Well, without more to go on, I don't see why it should be ASB.
Son of a lawyer - he has knowledge of the legal code and can become quite well-versed in law, an astute politician.
Country in turmoil and has executed its king, ripe for a revolutionary leader who is very good at passing laws and developing a legal code. So far it's actually quite *plausible*!
General: Mmmm, it's hard to say what makes good generals. Without more knowledge I can't say either way, there have been generals who have risen from nothing without going to a West Point or something, so as long as you don't make him the best general ever, still plausible, and even if he's a great general, possible.
Declares himself Emperor: Okay, there's a more questionable one, but they say power corrupts and absolute power corrupts absolutely. If he gets really power hungry, or even feels he needs to because of various threats...are we talking about Julius Caesar here, by any chance? No, wait, he wasn't Corsican, even ignoring the part about a country in turmoil.
Subdus the great states of Europe, now there's a place where I can see your point, it all depends on how ravaged by war they were before and how much of an advantage they had. If he had some great generals with him and far superior numbers, though, it's quite possible.
So, 1st 2, plausible, #3 and 4, a little less likely but still possible, maybe even quite possible, #5, we'd need more information.
Overall, I'd say if you want to put it in yoru TL, go ahead, but you might want to at least make mention of some real distrust of him once he becomes Emperor, maybe an assassination attempt even?
