It felt like David really wanted to do more with Harm, but just didn't. His first appearances were great, a couple solid fights, a neat design, and a dramatic ending with his father. I don't think it was until after this that David decided him and Secret were siblings, given how the Hayes parents are very clearly designed as Billy's parents, despite him supposedly being the adopted one. Anyways, he decides that Harm is Secret's brother, but wait! They never really interacted, and Harm is completely dead.
Very luckily there just so happens to be a big event where Hell freezes over and dead supervillains get to rise from the grave. So we get our family reveal and Harm comes back to kick the collective ass of Young Justice until Greta saves the day and gets to avenge her own murder, sending Billy to some unknown place. It's a really good issue, does a great job for Secret's personal arc, along with pushing the overall plot of the book forward, but I also think it's where PAD really started to change his direction with Harm.
In his first appearances, Harm was a gadget-based character, with a sweet super-car, a tricky knockout gas-dispensing sword, and the hacking know-how to hack Red Tornado (a feat that the nigh-omnipotent sentient AI known as Construct couldn't pull off). He did make a reference or two to "gifts" such as enhanced strength, but that's really glossed over, making Harm seem like a normal enough person who just so happened to have a special interest in murder and super-villainy.
With Harm's reappearance, his abilities were supernatural, due to being some sort of undead creature fresh out of Hell (complete with a new look and special word balloons and a demonic typeface just for him). Judging by later sightings of Harm, like his cartoon appearance and possession of his father near the end of the comic, I think Peter David decided that the supernatual approach to Harm was better.
That's a tangent, though.
Back to the point I started with, Harm feels underused. The Young Justice/Spyboy crossover uses Harm as its villainous representation, a position usually given to more overarching enemies (not that Young Justice really had any in its latter half), while Harm really isn't that present.
I think Peter David must've wanted Harm to play a bigger role in the finale arc of Young Justice, but just didn't have room for it. Bringing back a long-dead character the way he did, just to kill him off in between issues after only one or two actions doesn't feel right. I think one extra issue really would've solved the problem... If only we got to see just what happened while Secret was on Apokolypse
