Both Cornell and Carmilla acted civil towards Gabriel when they first met, but that doesn't make them any less evil. It's the same with Zobek, he was just being friendly toward Gabriel to manipulate the situation. I have no doubt that he "took care" of that child by killing him. Since he is the Lord of the Dead, theoretically every time a living thing dies, Zobek's power and influence grows stronger. The Dracolich was reanimated by one of his his generals. Just imagine how much power Zobek gained when Cornell, Malphas, Carmilla, and Baba Yaga were killed. Like dead puppets, he should be able to reanimate their bodies and use their powers as his own. The Lord of Death ultimately wants everything dead. Just because he can banter with Gabe doesn't mean he's not evil.
As for Satan, I think he made the classic villain's mistake of thinking he could use God's own power against him. He didn't understand that the Mask's power is more metaphorical than literal. If you could see what God sees, then you would theoretically reach enlightenment and peace. A perfect understanding of all things. This is why Gabriel saw Marie as alive while he had the mask on, because through God's eyes no one is truly dead. Gabriel was too crushed emotionally to truly understand what he was seeing though. And just like Gabriel couldn't see the truth beyond his sadness, Satan couldn't see the truth beyond his ego.
I personally don't think Gabriel became a Lord of Shadow at the end of the game. The three founders ascended to heaven, each with a piece of the Mask, when they reached enlightenment. But their more base and evil natures were left on Earth and became the Lords of Shadow because evil doesn't go to heaven. Marie tells Gabriel that he has a good heart and is not evil. But when she puts the mask on him, even though he is using it's power, he doesn't understand it and only sees disappointment. Gabriel isn't even split into good and evil halves. But Marie, who is technically Gabriel's better half, does ascend to heaven while holding the mask. Leaving Gabriel cursed to immortality until he makes up for his sins. Which is another reason why I don't think Gabe is a Lord of Shadow. He may be immortal, but he has the ability to redeem his sins. This is something the Lords could never do because they were sin.