Meta Knight is literally the Melee character of Smash 4. Once you're caught by him, you are as good as dead no matter what percent. There's ZSS too, but she's held back by her atrocious grab. I wished ZSS had a regular grab just for ***** and giggles. She and Meta Knight are the only characters who can instantly kill you if they catch you with their combos. Compared to Luigi and Ness who can drop combos even on fast fallers and heavies or Sheik who can hit you over and over, but if the player doesn't know how and when to kill with her, yeah...
Main reason why people hate Meta Knight is probably because his grab is good and second to Captain Falcon, Brawl, and well, he's not exactly eye candy to look at like ZSS.
People seeing Meta Knight at high levels was cool and refreshing at first, then he goes ZSS on you, but much more easily, and consistently. That same, stupid combo kills everyone. So, we hate this little puffball now. Oh, and probably PTSD from Brawl contributes to more hatred. Still, this could happen to everyone. Remember DK? Once Kong Cyclone happened, everyone hated the hell out of DK. Diddy is hated, Little Mac is hated, Link is hated, Luigi became hated, people hate Villager, etc. Every character is hated for one reason or another in this game and it's sad since they're from beloved series, but man, does Nintendo know how to make multiplayer so hateful. Play Smash or Mario Party and people are going to be raging. Now, I can't wait for a patch that makes it so Zelda can confirm Farore's Wind whenever.