I think there are people who definitely deserve to be executed. The problem I have is that I have zero trust in police or the justice system to handle it.

The police here in the UK are corrupt as hell, but it's ideological rather than fiscal corruption. They are very driven to get a conviction, any conviction, if they think the suspect is from the "wrong" side of the political divide. It took decades before the Guildford Four were finally released on appeal because the evidence against them had been faked (and possibly in some cases gained through extreme violence). The judiciary is almost entirely politicised, and convicts based on identity and which side of the political spectrum you are on.

So on balance, I can't support the death penalty until we are sure the justice system and police have been rid of corruption and political influence, and that the legal code itself is also revised to be non-political and applied equally to all. I don't see that happening.