Fine, Valmar then. Can't blame me for calling a character under a name they've been having for 5 goddamn years before RIot decided to butcher them. So "both are human" point is kinda ridiculous (even without that, as it encompasses majority of the champions). Now, let's see the differences:
-Kassadin still has hope left while Varus has none
One thing that completely differentiates them - while both lost their family members Varus did so to death and Kassadin to abduction. Thus Varus is a fully broken, bitter man out to revenge his dead family. Kassadin's ultimate goal is saving his daughter. Their goals, and this the methods they use to achieve them, are worlds apart.
-They didn't embrace their powers they were against "to revenge" (Kassadin's primary motive isn't revenge as stated above) in nearly the same way
Varus fully knew what the corruption will do to him, and came to the terms with that - as he has nothing to live for except revenge anyway. Kassadin, however, took many precautions to not get corrupted while harnessing his powers; it just turned out they weren't enough. He might be doomed, but if his precautions worked he'd still have a life with his daughter to look forward to.
-Aftermath and a character's influence on the world
Varus can be compared to a modern day radical terrorist; where he was done a great injustice by a certain nation, and focuses his wrath against the nation and it's people as a whole. He's certainly labeled as a murderer in Noxus and probably isn't welcome in other places either. And that's one of brilliant parts of his old lore - You know he's doing the wrong thing. You know he probably isn't much better than those who killed his family at this point. And yet you symphatize with him.
The power Kassadin is fighting is a great threat to the world as a whole. Even though killing as many Void inhabitants as he can isn't his primary concern, any damage he inflicts there is a service to Runeterra. Unlike Varus, the consequences of his efforts are heroic.
I'll say it again: Strip down the lores to their bare bones, and you can find an exact match for any champion. Analyze it a bit and you'll see it's not the case.
Oh, and Kassadin't lore got updated too. As you can see on his Universe page, his daughter is nowhere to be mentioned. He's now fully motivated by guarding the world against the Void. So, the "Varus vs Kassadin" point can't hold it's ground at all, seeing as Kassadin mentioned in that comparison doesn't exist anymore.