Ennis's punisher isnt void of emotion, he is pure emotion. He is a man fueled and entirely driven by rage. He has no define purpose and makes no attempt to justify, he's a force of nature. The lack of morality surrounding the character doesn't mean
he is unlike able, in fact there have been plenty of characters in literature that have been like that and totally engaging.
The problem is, existing adaptations of The Punisher focus on either trying to make him sympathetic or trying to exploit the violence. Both are flawed strategies. Ennius never did that, he didn't try to make the readers sympathise Castle, he challenges you to try and 'understand' him. His stories are like a psych profile, you learn what drives him and you learn his weaknesses. You also get a detailed look at the dark and violent world that has turned him into the Punisher.
That's what makes him interesting as a character and has just as much potential to work on screen as it does in print.
If anything Dexter is more screwed up because they try to make a serial killer appealing and quirky and likable.