I think it mostly depends on who or what the species is. For example, he always tries to save and protect humans no matter what. When they die it seems exceptional or they sacrificed themselves. The Doctor also tends to protect whatever species really need him like in Season 8 Episode 2, where he decided to help the Dalek since it seemed to be good. He also wanted the Dalek to be better than the Doctor since he didn't want the Dalek to destroy it's species with hatred. The Doctor basically helps all those who truly need him and doesn't really kill unless it is a threat.

I think it mostly depends on who or what the species is. For example, he always tries to save and protect humans no matter what. When they die, it seems exceptional, or they sacrificed themselves. 

The Doctor also tends to protect whatever species' really need him, like in Season 8 Episode 2, where he decided to help the Dalek, since it seemed to be good. He also wanted the Dalek to be better than the Doctor, since he didn't want the Dalek to destroy its species with hatred. 

The Doctor basically helps all those who truly need him, and doesn't really kill unless it is a threat.