I strongly oppose executing Julian. If he were mafia (or serial killer), it is unlikely that he would make a kill and then publicly announce it. It's too much of a risk and it's
not necessary.
The possibility that he's mafia would be substantially higher if it were plausible that he intended to make the kill secretly and subsequently believed that he might be caught. But according to at least meghal, she knew that he was talking to Jakob. There is no possibility that he intended the kill to be surreptitious.
Furthermore, as others have mentioned, there is little risk to keeping Julian alive. We just need someone to roleblock him.
If I'm missing something, someone please speak up.
Anyway (very probably) great job Julian!