Spoilers for VSMP Session 6 Apo's perspective.
I know this might not ending up being the case, but I like to think the reason Apo killed Truffle in front of Pyro is because she was hurt and she wanted to hurt someone else.
Like, Owen led them to the crypt where Scott and Pyro were waiting to drink from them, and in fact they both actually did drink from her. And then they gave her an ultimatum, sacrifice herself or someone else to be turned, or if she didn't, they would go after her loved ones. She chose to sacrifice herself so that no one else would be hurt and as a result, they're now trapped in Oakhurst, unable to see the very loved ones they were trying to save.
They're hurt, they're upset, they can't see the one they love, they're trapped in Oakhurst with the monsters that forced her to sacrifice herself pointlessly, because they ended going after the Townspeople anyway. She can't take all that pain out on Owen for putting her in this situation in the first place, she doesn't believe he's even capable of love so she has no meaningful way of hurting him, they can't take it out on Scott for being the one to turn her, they know he doesn't care about anyone on a meaningful level and thus there's nothing they can do that will hurt. And she definitely can't attack them, that's a fight they can't win and it'll only make the rest of the vamps go after them. But Pyro, who was also there, who fed on them, who loves Truffle dearly, can be hurt. Pyro can be dealt a fraction of the pain she is feeling.
So, she gets Truffle, a creature she knows Pyro loves, and gives them the same choice she was given. To hurt a loved one. And when they refuse, it only makes Apo angrier because how dare he act all high and mighty after taking her away from her loved ones. So she kills Truffle in front of them and gives him the corpse to eat. And immediately after she regrets it, because as much as she knows he also loved Truffle, she wants to believe he's a monster because of what he helped Scott and Owen do to her. She wants to believe he's sick and twisted. So, at the end of the day, as much as she hurt Pyro, they hurt themself much more.