During the climax of DOTM, Sam witwicky killed Starscream with a weapon given to him by que. To me; it was pretty FORCED on unattainable levels because in the last two movies, they had no real acquaintance whatsoever. Megs, Barry, Frenzy, Scalpel, and Rampage are the only cons Sam had any contacts with. Killing Laserbeak would've been enough. If you asked me, Starscreams death was a pure waste because he's punked Bumblebee and Ratchet in the 07 film and it wouldve provided a great impact if the tables had turned on him in the final act of DOTM while trading insults with his adversaries.
He blew up Bumblebee's legs, Sam saw how much in pain he was, he also shot down a helicopter with a soldier, who Sam briefly had a sigh of relief, from seemingly accomplishing his mission of delivering the cube, that risked his life, abandoning Bumblebee, Mikaela, Ironhide, and Ratchet to get that cube to the helicopter in 2007. Starscream also spit on him in ROTF, pursued him during the ROTF forest fight and Operation: Firestorm, feeling a sense of terror. And Movie version Starscream dying to Sam makes sense. Starscream is belittled by Megatron, so in turn he belittles Sam. It's a well-written end to the cowardly seeker.
Humans killing Transformers isn't that great but it does show how Sam has grown in this case. Frankly it's ridiculous to assume that a "grudge" that only you seem to know exists would be the motivation for a battle. Bayverse misses details it establishes sometimes, why would it rely on details that don't even actually exist in the movie?
This is all so inconsequential and minor, though. It pales in comparison to what other Movie 'Cons have done. Sam had never even mentioned Starscream by name, not even after he killed him. There was no indication Sam even remembered who he was.
In those moments he was below Starscream, under him, but in Starscream's death scene Sam got up in his face.
The movie handed the second most important Decepticon’s death to Screechy McTwat - it was too ignoble a death even for Starscream. It should have been someone like Ratchet; after all in Megatron’s absence it is likely that Screamer has been keeping Ratchet in business.