it's okay, everyone has every reason to like or not to like his decision.
and yes, it must be hurting most of the fan and the club also, they have done their best for hargo, they even keep paying full wages all these years, they spent for the medical, hargo only playing for several minutes, and now he's going to United biggest rival? of course lot of fans will hate him.
come on, put it this way, if he really has a good will, why these past several seasons he didn't initiate to propose wage reduction+salary per match scheme? I remember solskjaer did that. or maybe United already proposed but he rejected? and nearing end of his contract he's become panic and said to have proposed to play for fee, no wonder he'll make such a decision to join city.
when you said you love a club, if you ever join the biggest rival in future, it means what you've said is bulls*it. people will defend them with saying they need career, they need money, but with player like hargo that's almost reaching end of his career, surely he already has big amount of money, and if he wants, he can join any club except ones that hurt United fans, a disrespectful decision to fans who spent lot of money to support their club, not get paid to support, who's chanting your name in stadium, keep giving support.
yea, he was my favourite, surely i'll watch some City games if hargo plays, but rationally speaking, most of United fans surely will get mad at him. maybe they will say "i thought only your foot is made of glass, now i know that your heart also"

but, respect, fans still have reason to defend him and wish him the best, and hopefully will make it to England's squad, but never disturb United