You can always make a citizens arrest due to a suscpicion, however if you are wrong you open yourself up to criminal and civil liabilities. Police have inherent immunities from this, citizens dont. Nothing prevents you from trying however.
True, you can always try to make one. In this case the important thing to note is that it wasn't just the reaction of the suspects that mattered. If an innocent had been hurt the concerned bystander would have been in serious trouble, fortunately for him no innocents were harmed.
All he did was tell them to wait for police. they then initiated some form of contact, pulled thier weps, and got thier asses shot.
This is the part I've been having trouble with. Two criminals had their weapons out already pointed at him, then he drew and killed them both without a scratch? And you think asking questions about this story is unreasonable, don't you?
Lemme guess, you are a former/current LEO who thinks only government agents such as yourself should have the right to use force, and the rest of us peons have to wait for our knight-like overlords to show up, right?
I am not now nor have I ever been a cop, a security guard, a neighborhood watchman, or anything else related in anyway to law enforcement. All I am is a guy who asked a few questions which occurred to me after reading about an incident in Pennsylvania and I don't think the questions I asked were unusual or out of line in anyway. I would however prefer that civilians not try to perform law enforcement duties whenever possible. Sometimes you don't have a choice, we need more information in order to decide whether or not the bystander in this case was in that situation.