Iceman hates Havok. Havok and his leadership and his 'Lorna belongs to me' attitude and the 'I love Lorna' moods and the 'Why doesn't she love me?' days. Iceman always wants to go out and scream 'She doesn't belong to anyone' and 'I love Lorna, too' and 'She does love you, you idiot!' Iceman hates how Havok thinks that Lorna hates him. She just hates his leadership is all. Cyclops, his brother, was a better leader. Then he had to hook up with Emma Frost and become Headmasters and blah, blah, blah.
Iceman hates Havok's choices. He let Juggernaut become an X-Man. If he hadn't done that, Sammy wouldn't have died. Iceman knows that Nocturne and Juggernaut wouldn't have been sucked into Xorn's head, either, if Juggernaut hadn't tricked the Brotherhood. Iceman just hates the outcome of Havok's choices.
Iceman also hates how Havok screws up his own relationships. He screwed it up with Lorna, and just when he got a distraction, he screwed up his relationship with Annie, just because again, he let work interfere with his love life. Example—he let Juggernaut in. Iceman still isn't going to forget that one, for sure. So Annie went and took her son. Now, Havok's Lorna-crazed. Iceman hates how Havok left Lorna at the altar, and still thinks that one day Lorna and him will live happily ever after. That just makes Iceman want to stick his icy fingers down his throat and puke.
Only thing is—none of these reasons are liable to hate him.
Iceman hates that.