А у вас видеоигры слили!

For context, the implication is that Leo was showing Vito mercy here by taking him out quick, painlessly. The Commission wanted both Joe and Vito dead and in a realistic scenario, the closest person to either of them would've been the one to do it.

Joe is off to a fate worse than death, double confirmed by the unused text lines. He was gonna have a meeting with Mr Chu. To be tortured to death.

This ending is really grim, both characters we love - victims of criminal retribution, dark but realistic... and Vito's possible execution would be more of a mercy kill.