Oh, Winston knew exactly what he was doing. He just weaponized John for a campaign against the High Table. At the beginning of the movie, John just wanted to live. To the extent that he was willing to go back into their service, selling his wife's memory because then at least someone would remember her. Winston convinced John that it was better to die as a free man, but John presumably still wanted to either go out a blaze of glory or win peace and a return to normalcy. But now he's pissed off.Hey Winston, remember your quote from part 2?
Glass houses, man, glass houses.
And yes, I'm presuming that Winston wants the high table dead more than he wants to live. This isn't an assassination, it's a suicide bombing and John's the explosive.