Yeah. He already had a full head of steam, and had left his feet to try to stop Flacco's forward momentum before he got to the 1st down line on the 10. He led with his shoulder, not his helmet. I'm really not sure what you want Alonso to do there - just pull up and say "why yes please Mr. Flacco, go ahead and take the first down. Heck, have the touchdown if you prefer, there are no consequences!" Like, if that's dirty, maybe explain to me what the "good" defensive play looks like. From where I sat, Flacco wanted to have his cake (get the 1st down) and eat it too (have the protections of an early slide).
I've seen much dirtier hits than that, where there was no gameplay advantage to be gained or lost, just a defender out for blood. But the clearest category of them all are the plays that are after the whistle, which is what both the Ramsey and Evans plays were yesterday (and hence this sidetrack). That's where there can be no excuse for misconduct, and no room for argument by the player or the union. Suspensions for dirty hits during play are usually far less ambiguous than this.