Pros

+ No mana
+ Can poke
+ Insane damage
+ Strong all game
+ Gap-closer/escape
+ High skill cap
+ Flexible item build

Cons

- Weak burst when behind
- Loses to many tanky tops
- Death Mark is countered by
escapes / Zhonya's / QSS

You want to catch people out of position, you can do that by hiding in brushes where people want to go farm, warding up their jungle, camping their buffs and basically just play on their side of the map trying to find someone by themselves.

When you use Death Mark and the person is staying to fight you, feel free to use W and try and land a triple Q on that person. Usually that is enough damage to beat people who you normally wouldn't be able to. Just keep in mind, doing this basically means you're fighting to the death unless you have Flash.

An important thing to note during lane phase is when you're about to hit 6, if you can hit 6 before them and engage it'll almost always guarantee an advantage for you in some way.