I think in my god honest opinion, that West killed Kaito. Reasons why: He can fly, so he could've pushed Kaito off and flew away. He has the ski mask. He's the one in the painting making out with Claire and (possibly) could kill HRG. You can't say it wouldn't make sense that West killed him. Adam might have put him up to it to get revenge for his abduction. He could work with the Company. Who knows why he did it? But I think he did.