I got Lydia at about level 6.

Due to the way companions level, she was stuck at 6, as the Wabbjack glitch doesn't work on Housecarls. (Wabbajack transformations reset level or some BS.)

So, at about level 40, I decided I was tired of her. I disliked seeing her in my home, and hearing her repetitive quotes. Took her to a Forsworn camp in her normal gear and decided to let nature run it's course.

By that I mean, I soul trapped her and then murdered her in cold-blood.

She died the first time I took her out.

when Lydia dies, I always reload the last save, because I don't want to lose her.
and yes, I have plenty other housecarls (nope brought them all to the blades) but still, it's like Lydia is the perfect follower

You killed Parthunax?!?!

"You bastard!"
"JUSTICE!"
"Death to the storm cloaks!"

Hell, I never used used a companion once throughout the entire game unless it was absolutely necessary (like a quest where an npc follows you around or some shit).

I played a stealth character so having someone following me around would pretty much just blow my cover O.o