It was ABSOLUTELY for defense! Those soldier were ACTIVLY marching on his city with the EXPRESS goal of murdering EVERYONE.
The fact that their deaths would revive his friends was just a bonus. Which was ALSO justified, because they're the ones that KILLED THOSE FRIENDS IN THE FIRST PLACE! They are straight up being made to pay the cost to undo the damage THEY CAUSED!
They got off LUCKY. They were the worst offenders and got off with the lightest punishment.
He's tried repeatedly to get them to stop. It never works.
He's done nothing wrong.
Although he's probably giving Diablo too much free reign to operate without oversite.