Tarn is desperate to get bit by vampire!Megatron. Becoming an object of appetancy to Megatron's most base hungers is every one of his kinks — which he is mistaking for love — rolled into one and wrapped up in a sexy little bow.
Megatron finds Tarn's self-abasing desire to be consumed repellant and his ever-increasing urgency to win Megatron's favour pathetic.
And yet Megatron, in a moment of weakness, turns Tarn into a vampire. Then, being heartily annoyed with himself and taking it out on everyone but himself as usual, he immediately banishes him. He turns him loose to become a problem for someone else.