To put it as simply as possible, that guy, in order to keep his cushy government job, broke the cardinal rule of his profession and merged first his wife, and then his daughter with animals using unholy magic. THAT'S why he's so hated.
And, for reference, THIS is what she winds up actually looking like.
THIS is what DD posted in FB, when I reposted this: It's from an episode of Full Metal Alchemist; this guy creates a horrifying abomination out of his daughter and the family dog. You spent the episode getting to know and like the family before he does this and it's like a literal punch in the gut when you find out what he did. Most anime fans agree it "crossed the line" and hatred for that character and what he did is universal. Make sense?