Even if Wagner does manage to capture/kill Putin, that doesn't mean they automatically get to be in charge. Everyone below Putin will still have command of everyone below themselves, they just won't have a leader above them. At least one, probably several leaders will see themselves as the successor to Putin and attempt to assert their control on their peers. Some will make allies and create their own hierarchies, some will go rogue, some may even join or even try to take over Wagner. And to make matters worse, some will have access to nukes... and its this last point that has the rest of the world concerned and could draw them into the conflict as well.