It was a horrific situation and yes he is but as the commenter above was right that they have (or had maybe) diplomatic immunity. I'm not sure how all-encompassing diplomatic immunity is but I know it wasn't an issue when a guard with diplomatic immunity punched a reporter directly outside the White House during a White House press conference a few years back (iirc). They just revoked the diplomatic immunity status of that specific guard or guards.

When countries try to keep peace and nukes exist, sometimes things get messy and go without consequence, very unfortunate. I forget what else there is about the Saudi-America exchange, I'm not an expert in diplomatic affairs by any means, but I think any retaliation could lead to $20 gallons of gas and I think they might also own a significantly large percentage of our GDP debt. Do not quote me or take that as fact because I may very well be mixing up countries and information.