Tom isn't the ultimate villain of the story. That honor goes to El Patrón. But Tom is Bully Number One. In fact, he has a ton in common with another first-class bully, Draco Malfoy. Like Draco, Tom has it in for our hero, and he's related to some pretty bad people. In fact, based on what we see of him in the book, Tom seems poised to become a serial killer when he grows up, if his tendency to torture animals is any indication.
First, let's meet Tom. What exactly do we know about him? We first learn about him when Celia tells Matt that "Tom is Benito times ten! He can melt anyone's heart with those wide, innocent eyes. Everyone falls for it, but not me." Because we've already found out that Benito is not much more than a big fat jerk, we know that Tom is also a bully, and a manipulative one at that.
Later, we learn that he's got some violent, disturbing habits as well:
He looked past Tom and saw a frog on the lawn. Its hind legs had been nailed to the ground, and it flopped frantically, trying to escape. "You're disgusting!" Matt said. [...]
"I was only doing a science project," said Tom. (13.59-60)
We don't know what kind of science classes they have in Opium, but we're hoping they don't include torturing frogs. Here, and elsewhere, Tom is downright vicious towards others, especially Matt. The fact that María is supposed to marry him in the future sickens us (and Matt, too, for that matter).
But just what is Tom's problem? Why does he act the way he does? Well, a lot of it might have to do with Tom's status in the family. He's not really an Alacrán at all, even though El Patrón has "claimed" him. Tom is actually the illegitimate child of Felicia and Mr. MacGregor. Everyone pretends that Tom is the son of Felicia and her husband, Mr. Alacrán, but it's a lie. Tom isn't related to the Alacráns at all, which is a scandal he'd surely like to sweep under the rug.
A lot of people in Tom's family really don't like him, and this probably contributed to Tom's overall personality. We'd be tempted to feel bad for him, because he has such a rough lot in life, but that's no excuse for his own awful behavior. So in the end, we can't really blame Matt for hating his guts. We pretty much do, too.