Running into Officer Lamson with a speeding car, then blowing his brains out shatters any "moral boundaries," as it was unjustified. Rick is cop with cop driving experience, so he could have passed Lamson and quickly blocked him, much in the way Dawn's cops did the same to Noah. Rick was not interested in exercising that option, or waiting for Lamson to simply run out of hos own options, particularly since a person with bound hands cannot move as fast as one who is free.
That's probably the closest Rick has come to "killing for pleasure" rather than self defense but even then it's still, given the circumstances, on the side of it being a form of self-defense since Officer Larson was part of a group holding one of their people captive (Beth) he had just attacked one of their own (Sasha) and fled on foot, presumably or likely, headed back towards the hospital where he's clue them in on Rick's pending attack.
It's not like Rick was doing some kind of blood-sport thing where he set the guy loose just so he could run him down. This was, in effect, an escaped prisoner from a group of people who couldn't be trusted per Noah's tales.
So while he came "close" to killing for pleasure there it still, mostly, falls on a form of "self defense" or at least retaliation for him escaping captivity and no longer being "trusted" as a prisoner (who's to say he wouldn't attempt escape again?) so right on the line, but still not quite "killing for sport." The most "enjoyment" Rick got out of it was his "kill-quip" post-mortem.
That's world's different from the clear enjoyment and pleasure Negan was getting from mulling over who he would kill, and from killing itself, his whole group seems to take some level of "enjoyment" from killing. (See: the photos of people's crushed in heads on the wall of the Saviors' satellite compound.)