Latest and approved one: 
Who is he?
Montgomery "Monty" de la Cruz is a student at Liberty High and a friend of Bryce Walker and Justin Foley. He is a notorious and violent bully who is on the baseball team and serves as a major antagonist of the 13 Reasons Why series. Ironically, unlike most of the other characters he isn't even listed as a reason why on Hannah's tapes.
- He almost runs over Alex with a car,, as well as several other kids while speeding to school while speeding to school
- Beating up Alex
- Helping Courtney spread lies about Hannah being a "psycho lesbian"
- Ran Clay off the road, in an attempt to kill him
- Left a sex doll hanging on Jessica's (a rape victim) porch calling her a "slut"
- Mailed a loaded gun, as well as a shooting dummy diagram, along with a note saying " how could you live with your self" to Alex (a recent suicide survivor) Side note: Montgomery is basically trying to force Alex to kill himself, which makes him uncomfortably similar to Kilgrave and Valak.
- Wrecked Tony's car
- Left a crippled Alex in the middle of nowhere
- Broke into Clay's house and stole Justin's gun
- Left a dead rat in Zach's gym bag
- Vandalized the Bakers shop
- Left notes threatening to hurt or kill people who may testify against Bryce
- In the school bathroom, he beats up Tyler in a very violent way. Smashing his head on mirrors and sinks, dumping his head into the toilet repeatedly, and in one of the most graphic rape scenes he shoves a mop handle in Tyler's and sodomizes him with it.
Redeeming Qualities/Freudian Excuse?
He has an abusive father who at one point came after him with a hammer and had broke Monty' s arm after he acted up in school. Because of this he sometimes has to leave home. However I don't find this excuse mitigating, several characters come from abusive homes, that are just as bad, if not worse than Monty's. Justin who is a Foil to Montgomery comes from a broken home where his mother is a junky and his father walked out on him. The step father is also always abusing Justin. Justin himself became strung out on drugs because of this but he still has a moral consencice and didn't ever get up to what Monty has done.
Montgomery also is very loyal to Bryce even when the other jocks such as Scott were against Bryce for the rapes he had committed, Monty still wanted to protect him no matter what. However he is protecting Bryce because Bryce is the captain of the baseball team and without Bryce there season will be cancelled (which it was) so it is more to do with him protecting him becoming famous and protecting his social status. It is also never played as a redeeming quality, Bryce himself told Monty to fuck off since his Stupid Evil actions were putting his case at risk.
There is also this quote:
|“||Do you know what I got Alex? I got my friends, I got the team. I don’t get to be all boo-hoo I’m sad my friend died, I’m gonna put a bullet in my head so the whole world feels sorry for me. Jesus, you wave that gun around like I give a s**t. I don’t man. If I sell out Bryce or lie either way my life falls apart. I got no one else on my side. I can’t run home that team’s my home. Going to playoffs, winning state, being an athlete? That’s not about fun for me it’s about survival. It’s all I got. It’s everything.||„|
|~ Monty telling Alex his motives for his crimes.|
The show never plays this for sympathy though, and makes it sound kind of like he's using them as a means for his own survival rather than legit care about them...but they may just be my interpretation.
The only competition he has is the main antagonist himself Bryce Walker (who is Pure Evil). Bryce is a Serial Rapist but I'd say Monty still does enough to stand out enough from him. Monty's rape of Tyler is portrayed in a more graphic and painful way than Bryce's rapes were. I also think shoving a mop up someone's ass goes far beyond standard rape.
Also, unlike Bryce he has attempted murder and the mailing Alex a loaded gun part was beyond cruel. So yes, I think Monty qualifies because he pretty much doesn't do anything other than hurting others, and he has the most onscreen MEHs.