#To clarify. In the metaphor you are Kilgarrah and the hamsters are Merlin and co.

[robin-notquiteasbraveaslancelot]

I have decided that Killgarah's motives were, in fact, very justifiable, not only because he is a dragon, but also because of his age. Now, I do not remember if his age is ever directly stated in the series, and I refuse to either rewatch it or do research, so I am going to assume that he is quite old. Like, 1000 or so.

 Now, this is quite a large difference from any of either the characters he encounters during either the series or his lifetime, as the average person he would meet would be between 18-70/80. This would mean that, assuming he is not an outlier to dragon species as a whole, dragons live for anywhere between 800-1500 years on average. I am not an expert, but this seems reasonable? He is old, but he seems to be, for the majority of the series at least, in fairly good health, which would imply that he is not exactly in the last days of his life. 

As this has been established, we shall now move on to the main parts:

1) Dragons in relation to humans in general, with focus on Kilgharrah in particular.                                                  

Now, some of this will be speculation, but I think it probably would check out, taking into account that this is a fictional children's television series. The first part where this is a little bit speculation comes from the question: When is this show set?

 Now, there are many, many discrepancies in this show, such as the often mentioned tomato in the pilot to the fact that knights didn't really exist prior to about 800 or so, and the lack of Christianity means it couldn't have taken place after that. As this point is not key, we shall assume that this takes place in the year 500. Because I need a year.

So, that would mean that Killgarah was born in about 1000 BCE. Quite a lot was going on in this time, such as historical Israel was at its peak, the Assyerians were starting to do their thing, and the world population was about 50,000,000.

 From there to 500 CE, he would have witnessed the rise and fall of Greece, Rome, Persia (fall rather than rise, I think it had already risen), and countless people. He would have witnessed a massive chunk of human history, even if he was born slightly later than 1000BCE, such as if he was closer to 1000 than 1500.

Now, this would have probably given him a much different view on human civilization than a human, because it's hard to feel that individual civilizations matter when you've seen them all fall. Not only civilizations, but individual people would have probably started to be insignificant to him, as anyone he met would have died soon after, in comparison to his lifespan. Now, if this age thing is true for all dragons, then that would imply that dragons probably prefer each other much more than humans. Like, most people form closer bonds with other people than they do with say, a goldfish. You might really like watching the goldfish, but it would need 10 lifetimes to be equal to you in age. 

To conclude the Dragon's age section: People as individuals and as vast empires are basically meaningless to dragons, because they all die so fast it's not really worth forming a very strong attachment.

 2) Killgarah in relation to the prophecy

Now, this section relies on two main points.                                                                                                                        

a) Kilgarrah post the purge.  This is really quite tragic, as after this he has no other dragons left. He is the last of his kind, and if we take the previous ideas about the ages of dragons into account, then he is basically bereft of any meaningful companionship, and will spend the rest of his days alone, watching any human he might befriend die, leaving him alone again. Even if we assume that dragons probably don't form the same attachments to people that other people do, this alone would have not done wonders for his mental health and outlook on the world. 

 To top it off, he spends the next 20ish years in a literal pit, so he was probably not fond of people at this point of time. He does seem to get enjoyment out of Merlin's company, especially in the later seasons, so we can assume he probably liked humans a bit, but I don't think he would have liked them in an equal sort of way, as they were always going to be his inferior.  

Conclusion of this Section: Kilgharrah would have been unimaginably lonely, as his whole world (dragons) had just been killed.

 b) Kilgharrah and Merlin, and too many metaphors concerning hamsters.   So, one of the main things I see about Kilgharrah was that his advice was bad and that the goal of the prophecy was never actually achieved. Now, there is some truth to this, especially from a storytelling standpoint, but from the perspective of Kilgharrah, life did improve, and the prophecy was meaningless to him. Now, what Kilgarrah wanted was other dragons, and to get other dragons he needed a dragonlord. So, what he needed to do was to find a dragonlord who he could manipulate (Merlin) and who was not in danger of dying like everyone else in Kilgarrah's life (Merlin).  

This is where we speculate a little bit. Now, seeing as Kilgarrah knew about the prophecies and seemed to be able to predict destinies, he had the ability to act with quite a bit of foresight. We can therefore assume that he wanted the prophecy to go as planned, which meant that he never had any interest in doing what was good for people, only what was good for the prophecy. Why? Because if the prophecy gets completed, then he gets Aithusia, so he will not be alone, Merlin, so he will not be alone, and in a way, revenge on Uther, as bringing about he death of his son would probably be the perfect revenge as it was Arthur's birth that caused the purge that brought about basically the end of the world for Kilgharrah. 

The speculative bit comes from the question of: was Arthur living to build Albion part of the prophecy, or did the prophecy only want Arthur to make it to Camlamn so he could be killed by Mordred?

 If the latter option is true, then technically what Kilgarrah says at the end is true. Destiny was unavoidable, and so it was achieved, and Kilgarrah has benefited immensely. Now, onto the moral part of Killgarah's character, and why his actions were justifed.

1) He is very, very lonely. 

2)To not be lonely, he needs the prophecy to be completed, as he needs Merlin to hatch Aithusia, and also be immortal. He even says that the white dragon bodes well for Albion, and while it pretty much doesn't, it does bode well for both Arthur dying and Kilgharrah, which would imply that the prophecy wanted AArthur to die there, and Killgarrah knew it.

 3)While all this manipulation might seem bad, it makes perfect sense from Kilgarrah's perspective. 

To use a metaphor: Say you have a group of pet hamsters. You might really like some hamsters, but others you are neutral on. Regardless, all of your pet hamsters are only going to live for a few years, so you can't get too attached. 

Now, a person told you that you had to lie to your hamsters, who are slightly more intelligent than real hamsters, in order to save the lives of people.

 Now, you might really really love your hamsters, but you'd still lie to them to save people, right? You might not even know the person, but the life of a person is worth more than being truthful to a hamster. You're not even going to hurt the hamster, it's going to live forever, so what's the problem, really?

Now, say you weren't saving the life of a person. Say, all people you ever knew had just been killed. There's no more people, and you're just sitting in your house alone, knowing that you're going to live over the next forty years by yourself, sitting in one room, with nothing to do but think about the people you have lost.

Now, imagine you are told that there is a way to see another person again, that you won't be alone for the rest of your life, that you can have a friend. All you have to do is lie to your hamster, and get him to let another hamster die. 

Now, maybe you really love these hamsters. Maybe you don't want any of them to die. But you're all alone, and you just have to let one hamster die. The first hamster has a purpose now, and he's not going to die.

 Maybe a couple more hamsters have to die too, but you get to have another person again! Isn't it worth it? The hamster would die soon anyway, they don't live long compared to you.

 It’s not really bad, to let one hamster die, if it means you can have some of your world back, is it?