story identification - Dying person is the last in his race of warriors, trying to find the people who killed his race

GOT IT!

This is the Last Legionary series.

The protagonist is Keill Randor. The book Galactic Warlord covers the events you're talking about.

[Keill's] home planet was destroyed by a malevolent entity known as
'The Warlord' whose sole aim is to spread war through the galaxy,
enabling him to rule over the remains that survive. The Warlord moved
against the Legions since he perceived them as the greatest threat to
his plan.

The enemy you're thinking of was in fact "a shadowy force known as the Deathwing".

His sidekick is Glr, "a telepathic avian life form from another galaxy."

He is saved from his lethal radiation poisoning by "The Overseers," who "replaced his entire skeleton - using an unbreakable alloy". This plays into the conclusion of this book, where:

The Deathwing attempt to kill or disable Keill by binding him and then inducing convulsions strong enough to break his back. They believe they succeeded; however, his bones are unbreakable and they are deceived into thinking his back is broken and leave him unattended, etc. etc.

This is in fact a dupe in depth:

That said, I couldn't find them until I remembered and could search knowing the answer 8P. So I'm kind of filling this answer out with keywords in case it helps someone find it in the future.