The thing is, while AP malzahar is a very strong assassin with high scaling and lane pressure, after a while the tanks are too heavy, the carries have a QSS or the support's Crucible, and mages have Hourglasses. So you pretty much can't blow up people, but instead you will be focused and taken down easily. This is where AD Bruiser Malzahar shines. He can spawn voidlings once every 20 seconds in the early game and once every 8 in the late. You have constant source of damage and even when your ultimate is cleansed, you will still be able to survive the eventual burst put on you and redirect the voidlings with autoattacks or Malefic Visions

Pros
-Higher overall damage
-Higher survivability
-Higher pressure in waves and pushes
-Still useful in teamfights, once you locked down a carry your team will do the rest paired with your voidlings
-Somewhat hybrid damage, your skills still do a lot without scaling

Cons
-Less burst damage if you're not prepared with voidlings
-No chasing power (even with the Black Cleaver MS, boots and runes, if you catch someone with your ult voidlings won't arrive quickly enough to finish him off)
-Need territorial advamtage: you need to fight in your ground, with voidlings prepared and better if you're the one engaged. If you engage you need to be careful about the point above
-Teammates will most of the times react negatively towards a "troll pick" and won't fight at their best, looking towards the 20 minute surrender. So you need to show your worth and get the team believing, otherwise if you start feeding it will be downhill to loss.
-Countered by strong gap closers (Jax, Xin Zhao, Maokai...)

Voidlings can be quite hard to manage, since they work on priority targeting rather than your macro. Voidlings' targeting works like this:

1) The target of Nether Grasp
2) The nearest champion affected by Malefic Visions
3) The nearest non-champion affected by Malefic Visions
4) The current target of Malzahar's basic attacks
5) The nearest enemy unit.

Another thing to remember is that voidlings will evolve every 7 seconds, gaining 50% bonus AD and armor after the first 7 seconds (Growth) and 100% attack speed after 14 seconds (Frenzy). Ideally you wanna fight when your second voidling is about to hit Frenzy and your first has still 2-3 seconds of life left. This way your damage output will be maximized, and after the first voidling's death you will spawn another from the spell rotation you did to engage.

Remember to spam your skills on the wave or opponent as soon as you have them off cooldown if you are above 40% mana, that way you can keep the lane pressured and prepare your attack move. When below 40% limit yourself by casting Call of the Void sometimes on the opponent and using Malefic Visions when there are at least 7 minions (ideally stall a lane and when 2 waves are together use Malefic Visions on minions). You will gain the mana cost of your E by making it pass through 6 minions, so with this tecnique you can actually gain more mana than you started with. You don't have to kill the minions to gain mana back, only to let it pass, so if you're low on mana sometimes is worth sacrificing some creeps to let the Visions pass.

Keep an eye out for ganks: you are immobile if your flash is down and ganks until you have Manamune and Black Cleaver usually end up with you dead. If you're not used to low mobility champions, take some custom games and practise side stepping. It's litterally your best way of dealing with junglers.

If your enemy jungler has something you can't really avoid with simple side stepping then you need to ALWAYS have the river warded if you're on the blue side, and the tribush behind it if you're on the red side. Not only you will be able to see incoming ganks, but after reaching level 8-9 you will be able to turn a 1v2 into a kill and maybe a turret. Even though your abilities aren't your source of damage, you still have the longest silence applicable to enemies in the game, use it and you will be able to just walk away from a gank.

AVOID TRADING if your Malefic Visions is on a minion and the laner you are facing won't receive it when that minion dies, otherwise your voidlings will ignore you and keep farming. You can always use your ultimate to re-target the voidlings, but know that by the time your voidlings arrive to that champion Nether Grasp will be over, and you probably dead.

DO HARASS if your enemy laner is farming near a dying afflicted minion and you have a voidling up. Take even some minion aggro, you will usually be able to push your opponent to a forced recall.

IF FORCED TO TRADE kite back after placing Malefic Visions on the enemy and eventually silencing them. DO NOT autoattack, you can easily win trades if you just move backwards and let the voidling do the work. When Malefic Visions expires you will have to start autoattacking or voidlings will lose the target.

If your team has managed to keep an even score or even a slight disadvantage victory is assured. Keep your teleport up, farm waves near your team if possible, and if your team starts a fight FIGHT with them. If they're running away from a fight, use that time window to open an inhibitor, or if that has already been done, sneak a ward into the enemy base and start pressuring the Nexus Towers. With Zz'rot portal and your voidlings you won't really need a wave to destroy towers, you can decide to tank some damage and when your voidlings reach Growth go back and let them tank for you. If you manage to have at least 15-20 seconds of freedom victory is yours. If the enemy team backs you still have that ward you placed before: wait until they get out of base and go get your win.
Comment if you want to ask me anything, i will be glad to answer.