I made a new paladin build,but I can't think of a back story to suit him.I don't want him to just be a nice guy,I want him to be a vengeful nightmare against evil and the undead? I need a REASON to hate the undead (and various atrocities like falmer) Any idea how I can make a backstory to suit this?

When Your character was young, a group of thugs decided to steal Their Sweetroll, Your character threw the sweetroll to the ground and stomped on it, then beat up the thugs. Since this incident Your character has been plagued by an inability to eat a Sweetroll in Peace! The first bite into the Sweetroll will inevitably cause an incident that forces Your character into action, and the Sweetroll gets stale every single time! The desire to be able to finish a fresh and chewy Sweetroll in Peace has driven Your character to be proactive in killing Everything that may interfere with Sweetroll Nivana.

A Vampire killed his cat, Muffin.

He has seen what the undead, Falmer, and Daedra actually do to people, and knows the lies many spread to steal acceptance. Even for normal wicked people - Justice demands a hard line. Don't look away, don't blink, don't even wink in the face of injustice.

As for background - He'd probably be born into the upper middle class, or lower upper classes such as an impoverished knight - Low enough socially to resent those who abuse and misuse their power above them, accomplished enough to value the power of personal accomplishment and self-empowerment, with a strong enough sense of family/clan identity to hold himself above the "Common rabble" (Which he pities, not hates. Except when they do common-rabble things like kill, steal, and violate the law). He would not tolerate Injustice because People are Better than That, and need to Be Better. Also, a noble background also leaves him with a sense of responsibility for everyone 'beneath' him on the social ladder - it is his job to protect and defend them from the wickedness of Daedra, Bandits, and Necromancers.

Thieves, murderers and bandits are bad enough because of the damage they cause to the honest and just folk.

All of this said, he should not be ignorant or those he fights -
Banditry is unacceptable, because the effort and expenditure in arming and organizing, and time spent waiting and planning heists can be used more productively to create new, functional civilizations, or engage in honest labor to pay off mistakes from one's past.
Daedra worship - The 'benevolent' side of the most wicked daedra(Molag Bal, Mehrunes Dagon, Boethia, Mephala, Namira) is steeped in treachery, murder, and abuse, while the "Benevolent" daedra(Nocturnal, Azura, Merdia) require little digging to find an underlying rottenness. The "Neutral" Daedra(Sanguine, Sheogorath, Hircine, Peryite, Clavicus Vile), are all extremely destructive in their own way, and promote socially destructive behaviors.

Most of their portfolios are better handled by the Divines, anyway.

You need a reason to hate the Falmer? Blimey, you are a paladin

Is your character paladin-paladin, or more like Batman/The Punisher?

Just quickly came up this.

You and your brother are sent to kill a necromancer that plagues the small settlement called Riverwood. When you get to the entrance of his lair you both stop and say a prayer to the divines. After battling your way through many putrid undead abominations you finally encounter the powerful necromancer.

You exchanged words, allowing him to live if he walks away and never practice necromancy ever again. However, the necromancer declines your terms and in an instance, lightning shoots out of his hands! Your holy training made your reflexes quick, you raise your shield to protect yourself, bolts of lighting arc over you.

Unfortunately, your brother was not as quick and he was slain almost instantly. As you remember your training of mind over body you swallow your grief, but you cant help but let out a cry of anger as you charge the cackling necromancer.

You slash down, certain that your sword is going to cleave him in two...but it doesn't, a magical ward spell pushes you back, dazed and confused.

The Vile necromancer scowls wickedly at you and then grins a vulgar smile. From the corner of your eye you see your brothers lifeless body standing up again, but he's not the same as he was...his limbs look weak and lifeless, almost like a child's doll.

The shambling corpse steps closer to you, sword raised and ready to attack. Refusing to fight him, you block his attacks with your dented shield. Again and again he batters your shield with lumbering swipes until you are too fatigued to fight anymore, you fall to your knees and the sword cuts through your armour.

You fall to the floor, wishing that this was just an aweful nightmare...but it's not. The necromancer curses you saying you are a weakling. He raises a gnarled finger and beckons your brother to follow him and they leave the cave.

But you are not a weakling, you are the god chosen holy warrior. Miraculously, you stumble to your feet, the necromancer and your brother are nowhere to be seen. You clutch your deep wound and stagger into the fresh air. But it seems this nightmare is not over yet as you are grabbed from behind, you recognise thees people....they are imperial legionaries. They tend to your wounds and help you up into the back of a wagon.....