誰か ... 誰かがやったことにするんだ ... ！
I'm guessing this is essentially, "Someone ... someone [else?] will have done this," as in, he's going to pin the crime on another person.
Yes, that's right. Literally it translates as "I will (んだ) make this into something (ことにする) that someone else (だれかが) did (やった)".
I'd like to make sure that I'm understanding 「やったこと」 ...