I think its just who he is really and also that he has been a double/triple agent for voldemorte/dumbledore for so long he says one thing and totally means and does the complete opposite...so maybe just holding up pretences for everyone's lives and his...

As for harry...he may despise harry cuz of who harry's father is and harry represents the man who married the love of snape's life but yet he may be cruel to harry but he kept to his vow and protected harry

I think that, for example, about harry, he just protect him for still be Dumbledore's "friend", but don't talk good about him because he was in the side of the dark lord. And also because he ate his father.

True.
That's the essence of being a spy. Words and deeds do not match.
Only the true, deep motives are important. In Severus' case, these shows his real agenda.
However, in the light of his motivations for action, he acts in accordance with his objectives.