Pros:
+ Huge push power
+ Great for harassing your enemies
+ Ultimate is viable in team fights
+ Once you have enough gold, you can spam your abilities

Cons:
- Slow movement
- Low health
- Low Mana at the beginning

Malzahar is not an all-in champion. His playstyle is about making damage over time. This means most of the time you flee from your targets and harass them while fleeing. Once they their HP is low enough you finish them with a Combo of E>W>Q>R. His Ultimate is very useful in teamfights. His Q can be used to make to slow down or stop even large groups of enemy Champions.
Your passive is very useful in fights, because it absorbs 90% damage and most stuns and debuffs are ignored, but it takes a long time to recharge. Disengage from fights and wait for your passive to return.

Level 1: Stay back and wait for Level 2.
Level 2-6: You can start pushing. E and W, will give you enough power to push the lane. Focus on killing the minions which you hit with your E. Because everytime one of them dies you get a little bit of mana back. Pushing too hard can put you at a huge disadvantage. You will be open for jungle ganks. Ward one side of the river and stay there. This will give you more time to react if a jungler comes to visit you.
After you've farmed 1300 Gold, go back and buy Lost Chapter. This will give you enough mana to start harassing and finally use your Q.

Start harassing. Stay on your lane and try to get the first tower. After that start roaming and help your teammates. Your Ultimate is very good in those situations. For your Ultimate always choose the enemies with the highest DMG output. Combine your Ultimate with your Abilities. Use E>W>Q>R for maximum damage. Priority targets are ADCs and Assasins. Don't try to fight tanks unless you have a level advantage.

Stay with your team. Don't rush in fights head on. Stay back and harass them from afar. This is by far the best option for Malzahar. Try killing dragons. Most of the time the dragon will attack your voidlings instead of you.