Low HP isnāt necessarily the determining factor for how durable a hero is. Defense is more important; as the attack forumla in Damage Calculation notes, higher defense can greatly decrease the damage taken per hit (thus expanding survivability of a hero exponentially).
But that aside, Lepus is not a defensive hero. Heās a glass cannon, in the truest form. Low HP, mediocre defense, but very high damage output with fast speed.
When you take Thorneās skill, but give him the same speed and skill power as Magni, you get a hero that is quite destructive. I think he really needs that durability drawback, as heād be far too overwhelming without it.