auto attack

Is there a way to auto attack where your main character/hero will attack a target until it dies, then automatically move on to the next target and attack? Thanks.

No, atleast not any way that wouldn't break the rules.

It's called aggressive mode for heroes, they'll attack any target that move into range and keep fighting until everything is dead. However, this doesn't do anything for targets out of your aggro radius.

My question is in regards to the character that I control. What's the command or buttons to activate this aggressive mode?

You can press spacebar. Your character will then auto-attack the foe you had targeted when you pressed spacebar. If you're melee you will follow them and auto-attack, and if you have a staff/scepter equipped you will auto-wand them etc.

I hope that's what you meant.

Also, i don't know of any button that makes your character move on to the next closest target once your first target is dead. There is a button you can manually press to target the next closest foe though.

Aggressive mode is only for your hero companions.

There is no way to auto attack where your main character/hero will attack a target until it dies, then automatically move on to the next target and attack, other than downloading a botting program and getting your account banned when using it.

Use Options to rebind Target Closest to Tab. The Tab-Spacebar will keep the attacks rolling. But you must press this key pair, not a bot program.

Heroes only. The controls are activated in the individual hero control panel (click the grey icon next to hero's name to open, then look in the panel for three icons that set them to aggressive, guard and avoid combat.

But you won't get to automate the game that way, btw.

pressing the C button will auto target the nearest foe to you

C-Space.
Twelve.

I often use tab+space so i can get to target a enemy healer or caster instead of mindlessly just hitting nearest enemy who most often is a melee or minion.
But you must manually attack the next target once the first target is dead.