For instance, when Potter was trying to do a spell in class, Draco discreetly made it look like Potter did the spell first try - and so Potter would get points for Gryffindor ... for something he didn't do. Draco knew the guilt would eat away at him, so he would do it as often as possible. How truly diabolical of him. (Harry really didn't understand why Malfoy kept helping him, but had no interest in stopping him. After all, free points and he would learn the spells anyway)
Potter was unaware of the first task, huh? Well good thing Draco was there to spoil it for him. Draco made sure to tell him all about the dragons, and the details of the task (that he had stolen from Dumbledore's desk). Now the surprise of the first task was spoiled for Potter, and he would never get that first time reaction to the task. He would also be panicking over the thought of facing dragons. That'll show him. How extremely nasty of him. (Harry was honestly grateful for the information, even if Hagrid would give him the information a few days later. It gave him a head start on preparing for the task. He was just confused as to why Malfoy kept helping him despite hating him.)
Potter and Daphne Greengrass clearly being into each other, but not allowed to be together by Daphne's parents? Draco was able to trick them into accidentally signing a marriage contract, so now they were FORCED to be together. Their relationship was now unnaturally made, with their choice taken away from them. Draco knew they would grow to resent and hate each other. How legally evil of him. (Harry and Daphne had been secretly dating for a while at this point, so they just shrugged and went along with it. After all, it was an easy excuse to openly be together.)
Potter had the Dark Lord as his arch-nemesis, and was looking to fight him in a epic final battle? Not on Draco's watch. Draco gathered the horcruxes and captured Voldemort & Nagini, and had them tied up in a unbreakable room. Then he tricked Potter into casting fiendfyre into the room. Now Potter couldn't get the glory of killing the Dark Lord in front of everyone. He would always know that he beat a guy who was all tied up, with absolutely no effort whatsoever. Potter would be devastated at the missed opportunity to show off, and would be upset and filled with shame. How ultimately horrible of him. (Harry was really confused as to why Malfoy had basically done all the work, but he didn't really care. He didn't want the attention after all, and this avoided a lot of people dying.)
Potter and Greengrass were going to get married, huh? Would be a shame if Draco ruined that. How about by revolutionising a cure to Astoria's blood curse, requiring a ritual to be used which just so happened to be only doable on the wedding day, forcing them to delay it. Now their perfect wedding was ruined, and it could never be perfect. Draco had won. How amazingly monstrous of him. (Harry and Daphne were relieved at Astoria being cured, and were more than happy to delay the wedding for this ritual. After all, the wedding could happen anytime. Saving family? That could only happen now.)
Oh, were the Potter's now trying for a child? Well Draco had plans to ruin that for them. He would discreetly drug Daphne Potter with a fertility potion making it easier for them to have a child. However, he then done it again and again and again - thereby they would have loads of children like the Weasley's. The shame and embarrassment of having a family that big would haunt them for the rest of their lives, alongside the disgust of being compared to those disgusting Weasley's. How biologically terrible of him. (Harry and Daphne actually wanted a big family, so this was truly a blessing for them. Even if they had no idea of Malfoy's involvement at all. They also actually liked the Weasley's, and unlike Draco Malfoy they had no misplaced feelings of inferiority towards them.)
The Ministry was making a monument dedicated to the victory over the Dark Lord, and Draco knew that Potter was going to want it to be all about him, like the glory hound he is. So he went out of his way to ensure the monument would barely have Potter on it at all, and would have the names of everyone who died during the war. Potter couldn't hog the attention now, and would be forced to see all those who died because of him. How universally malicious of him. (Harry preferred it this way. The Ministry originally wanted a statue of him, Ron and Hermione. But none of them liked that idea. A monument with the names of those who died on it would mean they would never truly be forgotten, and would be about them, not him.)