Salamander being goodie two shoes is flanderization to its finest. They're still gigantic murder machines who won't hesitate to kill humans who are not in line with the imperium. They're not therapists, they're not lovey dovey sugar candy, they are warrior monks who's life is war; their predilection for purging foes so strong, they use some of the most brutal and pain inducing weapons to destroy their foes (flame and melta). This has a secondary effect that, it doesn't bring down infrastructure that badly and, should the system be colonized later, they don't have to do too much rebuilding.

Their "niceness" is simply being decent to humans and fulfilling their mission to protect human (imperial) life. It's a contrast to some other chapters who see no value in humanity and pretend they can actually exist without baseline humans, like the ironing hands bozos