I do believe he had good intentions when he possessed Sam. He used up his own energy to save Cas and Charlie, when he could have saved it to heal himself. And the fact that Sam is mostly healed at the end of 9.10 indicates that he's been doing some work while he was in there (although it could have simply been a plan to get that tasty meatsuit for himself).
So, let's look at Gadreel's good and bad side:
- Healed Sam
- Saved Dean
- Resurrected Castiel
- Resurrected Charlie
- Put Sam in a dream instead of letting him experience possession and torture
- Let Lucifer into the Garden (truly bad or just tricked/misled/naive?)
- Coerced Dean into kicking Castiel out of the bunker
- Lied to Dean
- Killed Kevin
- Killed Abner
- Stole the Impala
- Refused to leave Sam
- Followed the wrong person. And unlike Sam, he didn't have someone he should have trusted telling him not to.
- Did the wrong things for what he believed were the right reasons. Yes, it's horrible that he killed Kevin and Abner (Thaddeus, not so much). But he thought he was saving his people. If I can forgive Sam for killing Cindy the nurse, why can't I forgive Gadreel?
- Didn't question the one who encouraged him to distrust/abandon those he trusted most.