The Doomslayer

While it might seem obvious, people might not help making a killing machine hero.  You don’t want them to come off as villainous or bland.  It is possible because there’s a secret balancing act between the violence and their humanity.  Otherwise, they’d just be a brutal monster.

1. Remember that people can become desensitized to violence and gore.  If you keep trying to one-up yourself with every kill, you’re eventually going to hit a wall and people will get bored.  Sometimes, it’s okay to keep things simple, but you also want to have long spaces between acts of violence.  Let the audience relax and return to their baseline.
2. Keep in mind that these characters are different than pure berserkers.  While both unleash death and destruction, the killing machine isn’t mindless.  They are still in control of their thoughts and actions, which is why they are able to do rather creative attacks.  It isn’t bashing through blindly here.  They rack up a high body count in a fight by doing a controlled release of their anger and come very close to losing it completely.  This means they need at least a sliver of control when going wild.
3. Give the killer machine hero other things to do when they aren’t in combat.  This only works if they exist in a world where non-violent activities are allowed.  So, Doomslayer pausing his battle in Hell to do a crossword puzzle won’t work.  For those who have the option, give them hobbies and interests to humanize them.  You don’t want them to be defined solely by their berserker rage.
4. To make their outbursts more impactful, their tempers can either start out or become longer.  Instead of having them go on a killing spree over the slightest annoyance, give them self-control.  This keeps them in the hero category too because they need a reason to go wild.  A great example is Marvel’s Wolverine who started out with a nasty temper and evolved to be more mellow.  I remember a few storylines where he learned to be more calm or found ways to permanently release his pent up rage.  The trick here is to keep them a killing machine, but make it situational.
5. These characters don’t always have to be gruff and dark.  Yes, they might be hiding a dark past that makes them lash out.  That doesn’t always translate to being moody and growling at everyone.  In fact, a happy and friendly character abruptly releasing their rage on enemies would be fairly shocking.  This isn’t even weird because some people hide their pain through laughter and friendliness.
6. Please make sure they’re only killing bad guys.  Once they take out an ally or an innocent bystander, these characters either go villain, collapse from remorse, or willingly go to jail.  Remember, they still have a sense of morality and wouldn’t want to hurt someone who doesn’t deserve it.
7. Give them friends to keep them centered and human.  A killing machine being solitary increases the chances of going over the edge.  Friends, family, loved ones, pets, or anything that will make them care.  Give them the capacity to think of others in order to grant them a positive, gentle side to their persona.