Malzahar going AD in long term is noob.
There is NO effect for building attack speed and critic chance for your Voidling (that insect pet won't benefit much).

On the other hand, you can see the +% AP ratio is outstanding high, but NOT AD.

so, never never go AD in your item build !!

In early game, AP Malzahar is weak in jungling and mana hungry.
You lowering your jungle speed and quality of ganking.

Therefore, you have to get some AD what can effectively doubling your AD output by the pet.

Jungle is a special position and Jungle Malzahar is even more special.

OP champion and OP Ultimat
=> absolute a kill for every gank.
=> Powerful in late game and teamfight
=> good backline protection if enemy have good assassin in teamfight

You want to carry a game
- AP output builds carry to win

Someone (noob) get your Mid-lane
- you can play AP carry now

You see all teammates (noobs) going all AD (or lack of AP output)

Pros

+ Guarantee success gank if Ult can place + outstanding player use it can carry the game + Good protection to ADC in team fight. +

Cons

+ Malzhar jungle need good skills (don't let your pet die too early) + Mana hungry and needs to caution anti-jungle. + In a lose game, easy to be blamed by others (by noob teammates)

First item for Jungle Malzahar. Because of the mana issue, having this even before the boot is ok if second back have enough money. It is because the gank of Malzahar is ultimate but not to chase. If you have no mana, you cannot even try gank.

Of course, AP carry needs Magic penetration. But it can be upgraded after zhonya's hourglass

Malzhar is squishy, as a squishy jungler, better to have armor. Also, malzahar needs AP, therefore, i perfer hourglass for third item

Team fight time, AP carry suppose to sit back and poke. you always face tank enemy instead of backlines in normal case. Magic Peneration is more important

For a carry (AD or AP), Fifth item is defensive is crucial because somethimes your taemmate cannot protect you so much and your needs some defense

good item for increase damage, but the 6th item is seldom getting