Subject: Re: SCOTUS finds immunity for offiical Pres acts
Obama gave the order for drone strikes, and as a result an American citizen died.

That's not sufficient to charge a crime. You'd have to prove he ordered that specific drone strike against that specific person to support a premeditated murder charge. Even to get to conspiracy to commit, you'd almost certainly have to prove that he knew that al-Awlaki was on the target list (which apparently he wasn't), or at a minimum that the President knew that American citizens were on the target list. There's no indication that's true.

There's no statute of limitations for murder.

You can't charge Obama with murder. He didn't kill the guy - the drone pilot did. Probably the strongest charge you could get - if Obama was actually involved in that specific strike - would be a conspiracy to commit murder charge. And that's going to be barred by the statute of limitation.