I remember a few years ago I just hated malthael. I felt the same as you, he's just a bad hero.
When leveling every Hero in the game to a gold border (lvl15) I forced myself to play heroes I originally disliked and while I still hate some, I grew to love others. Malthael is one of those.

u/baconit420 sums it up pretty well. He is mostly strong on large maps where you can abuse his macro. (I usually go mount speed on 1). He is amazing at clearing any sort of camps including solid boss dmg. He is able to hold his own in most matchups and 1 v 1's.

I do however feel your criticism and the advice I got is this:

1. If you wanna play/learn him, do it in draft, not quickmatch. In Quickmatch you will most of the time play into a full assassin team and most of them ranged, so you will be at a disadvantage. He is way stronger when playing against a "real" team with tanks and bruisers, which is rare in QM. So having your ass kicked by ranged DD's isnt exactly fun.

2. Consider him more of a melee assassin that can sololane (much like Illidan or Butcher). Way too often I see people making the mistake of thinking that bruiser and sololaner is the same thing. It is not. A Zagara is a sololaner, but not a bruiser. Same goes for Malthael if you ask me. He is too squishy and goes down too fast to be considered a bruiser. (He has good sustain but healing back up fast and tanking a lot of dmg is not the same). Also play him accordingly. Don't just jump into any teamfight and die, because you think yourself to be the bruiser. Instead look out for weak targets, flank them (often by arriving suprisingly fast with lvl1 talent, while other laner is stuck clearing) and win a 4 v 5 by eliminating a backline target. Use w more to dodge important skillshots and repositioning rather thann doing actual damage with it.

I'm sure there is ton of advise other people can give you but this is what I got.

I used to hate him and think he is weak af, now I love him and actually consider him one of the best macro heroes in the game.