Everyone's favorite Space-AIDS-throwing Prophet of the Void has always seemed so tragic to me. Do you think he could ever go back to being the man he used to be before the Void got hold of him? Is he merely 'possessed' by the Void, so he could return to normal if it were pulled out of him? Or is his mind completely broken, so that removing the Void would only take away his power, not his insanity?
Are you sure he's insane as he is, currently?
rejected the Void because its incoming invasion will change life as Runeterrans know it; 
embraced it because the Void is an immeasurably powerful dimension filled with an untold number of malevolent, often-intelligent creatures, such as 
and 
. What chance does Runeterra have? No one knows, with a few possible exceptions (, 
, ). Malzahar simply believes that the Void will win (and he's one of the privileged few that have a good inkling as to whether that is true), eventually, and he's immortal. Why not be on the winning side, even if it's full of hungry monsters?
I believe Malzahar succumbed to the visions of armageddon promised by the Voidborn, not too dissimilar to Saren and the Reapers.
There's a choice, really: you can have him be saveable or not. Both are very valid and can lead to different kinds of stories. I suspect he's built to fit the Saren role more than anything, so a tragic ending may be what is in store.
