Dreadwing isn't a character I particularly resonate with for his own sake (like his personality doesn't do it for me), but he was so great for what he revealed about other characters.
So, Dreadwing is a foil to Starscream. Starscream is sneaky; Dreadwing is forthright. Starscream will do anything to survive; Dreadwing is all about honor. Starscream will sacrifice any other person / relationship to get ahead; Dreadwing puts his life on the line to try to avenge his brother.
And finally, Starscream is good at his job, Dreadwing isn't.
This is one of the most interesting things about Dreadwing. He's buff, he's serious, he's honorable, but once you start looking at his track record . . . hoo boy.
Mission to get rid of Airachnid: Dreadwing loses control of Breakdown, allowing him to be goaded and lured away by Airachnid, who kills him and traipses away.
Antarctic mission: Dreadwing returns emptyhanded while a rogue Starscream escapes with the Apex Armor.
Omega Key mission: Dreadwing gets knocked out by his own bomb; Starscream clocks Smokescreen from behind and steals the Omega Key.
So let's fast forward to the episode "Regeneration". Dreadwing has just learned that Starscream turned DW's twin into a zombie with dark energon. Megatron tells Dreadwing to get along with Starscream because the 'Cons can only succeed if they're united.
Dreadwing is like "okay" and immediately goes to the med bay to kill Starscream. While Starscream is cowering and pleading, Megatron storms in and orders Dreadwing to stand down. Dreadwing refuses but right before he can kill Starscream, Megatron shoots a hole straight through him.
So, Megatron would have preferred to keep both Starscream and Dreadwing on his team . . . and IMO he did initially prefer Dreadwing's vibe to Starscream's. After all, why wouldn't an ex-gladiator prefer the hulking warrior who has the strength and confidence to directly confront his foes, over the lanky, conniving guy who has to plot in the shadows and backstab to accomplish his goals?
But when he only had a second to decide between the two, Megatron shot the big buff guy who kept failing missions and was actively disobeying him, and saved the skinny sneaky guy who--even hobbled by the loss of his t-cog--had succeeded in obtaining four important relics and who could (somewhat) be controlled.
Dreadwing might have been a better person than Starscream, but Starscream was a better Decepticon. And making Megatron realize that was the most significant thing Dreadwing ever did on the show.
