Normally Nasirov would never be convicted of anything by Ukrainian courts, but the IMF, EU, and America see this as a test case as to whether Ukraine can deal with its corruption problem. Continued foreign aid might depend on Nasirov being convicted. Poroshenko has an interest to make this happen.
On the other hand, maybe if Nasirov sees that the government truly plans to convict him and give him meaningful punishment, he wants to cut a deal. He might have information that could take down higher-level officials (maybe even Poroshenko himself) or important oligarchs for their own corruption schemes.
Put it all together and it could benefit a lot of parties if Nasirov were to somehow end up dead. The foreign benefactors would have no reason to withhold funding, and the local thieves remain safe. Maybe I'm too cynical.