What perk trees do I need to spend points into to become a "assassin" ? Sneak? I was watching someone play and they where doing crazy sneak attacks that 1 shot stuff and the cinematics look amazing

There is a sneak perk that gives you x15 damage multipliers for sneak attacks with daggers.
The entire sneak tree is key. You need it for that damage multiplier, and you need it to successfully sneak up to people in the first place.

After that, you do your best to increase your damage with daggers.
Get the best possible dagger.
Take the first perk of the one-handed skill (Armsman) to increase your damage by up to 100%.
Take blacksmithing and craft the best dagger for yourself, and sharpen it to increase its base damage.

Enchantments and poisons unfortunately do not have their damage multiplied by x15 during sneak attacks.

Do you have to be behind the target to get the 'backstab' bonus?

And do power attacks count as sneak attacks when hidden?

Yes, on both.
You don't technically have to be behind them as long as they don't see you, but thats hard to do.

The only time you get the 'stealth kill' animations though is only if your damage is going to exceed their hitpoint total.

Assassin is a vague term. Take an orc with a giant war hammer, 2h perks, loads of health and stamina and you'll have a very good killing machine

If killing is your primarily goal, you could also try several unconventional builds and see which one is more efficient.