In Harry Potter and the Prisoner of Azkaban Prof. Lupin turns into a werewolf and tries to kill Harry and the other day Harry talks nicely with Prof. Lupin.
What was that?
Movies & TV Stack Exchange is a question and answer site for movie and TV enthusiasts. It only takes a minute to sign up.Sign up to join this community
Because he knew that Lupin is a werewolf because of Hermione.
Hermione: He's a werewolf! That's why he's been missing classes.
Lupin: How long have you known?
Hermione: Since Professor Snape set the essay.
A werewolf can hurt his dear ones without knowing and transforming into a werewolf is very painful too. Since Harry knew he is just having a rough time being werewolf, he talks to Lupin normally.
Obviously, if someone's dear ones hurt him/her while being in trouble and he/she knows that they are not doing this intentionally, he/she would talk normally and help'em.
Hermione and Harry later in the forest after they freed Buckbeak:
Hermione: Poor Professor Lupin's having a really tough night.
Because a werewolf has no control over themselves if they turn.
However, some of the worst effects can be mitigated by consuming Wolfsbane Potion, which allows a werewolf to retain his or her human mind while transformed, thus freeing him or her from the worry of harming other humans or themselves.
But Lupin had not taken his potion that day. So, he lost control.
Lupin was a good person and he was turned into a werewolf by Fenrir Greyback (http://harrypotter.wikia.com/wiki/Remus_Lupin) at a very young age, that wasn't his fault.
Moreover he was Harry's parents very close friends.