It takes so long to show proof and build a consensus (if one can be built at all) the perpetrators are pretty much beyond any punitive action. Right now they can depend on this, to the point they can do a cost / benefit analysis.
It's time to put a little uncertainty into their lives. A little swift, unilateral action will screw up their calculations and make ROI far more uncertain.
Ok, there's the risk we might kill some criminals that aren't the ones that actually did that particular job. I'm willing to accept that risk.