I really enjoy this skill as a ranger, I think it has a lot of potential, though it kind of feels bleh atm. Although I'm aware that most of us probably run tiger...I think you could improve this skill by adding in some changes such as:

If the marked target dies while under the effect of the mark, the ability’s cooldown is instantly reset. (To prevent this from being overpowered in prolonged fights, the reset mechanic could have a cap.)

Or to add more pizzazz you could add in a:

Explosion Effect: When a marked target dies, it could deal AoE damage in a radius around the target by either storing the "extra" damage your abilities do, or either debuffing the targets around with some sort of slow/shaken debuff(since their buddy just exploded)...

Also the level 10 ranger passive feels pointless...(just give us an iddy bitty self heal)

Thanks for listening to my Ted talk.