You meet your evil self from another timeline... What do you do?

I'm only 20% good, so they're 80% good. Murder them and take their stuff. It should be easy, stupid goody two-shoes.
- This was my first thought. My opposite will be gullible and am easy mark.

I'm pretty sure I'm 55% evil so he would be 45% he and thus the good one.
I'd probably take a car to our mum's. Kill Phil, grab Liz, go to the Winchester, have a nice cold pint, and wait for all of this to blow over.

Some guy pops out blabbing something about being the good version of me and I continue eating my sandwich
- They make themself a sandwich with what you have left, eat it, and leave. You get a Venmo payment for the cost of a loaf of bread and sandwich ingredients later.
  Truly, the good version.
  
  Good me definitely also understands how important sandwiches (and sandwich precursors) are to us so that tracks

I would fuck my self
- Truthfully I’d be super curious what a fully unlocked evil me was capable of.

Are they also my opposite in how they are good or evil, or just opposite in quantity?
For example, if I'm an antisocial asshole with positive ideals, does this mean my opposite is an extremely friendly and polite business major?

as someone who is 100% evil but polite, it would be interesting to meet my good counterpart whom is a total arsehole.
- What do you think makes you evil?
  
  I wish I could tell you

My evil self would've already completed her villain arc in which all of her art would be Pikmin fanart, except it's actually Breaking Bad memes in varying degrees of subtlety.
So I'd become her sidekick and do the same. I regularly pretend to fight myself in the mirror so that I can get really good at fighting with any and all potential clones of myself, so on the off-chance she wants to try and fight me, I have the experience.
- this would make a good sitcom, something in the style of Reno911

Get someone to do a very long set of interviews, thought experiments, and tests to see what makes us different in order to isolate, regardless of whether we can tell which of us holds the evil position, what areas of belief can have a position than can be objectively called evil. It'd be hilarious if, after months of testing, it turns out the only difference we have is our opinions on marmite or pineapple on pizza.
- That's a smart move. But won't your evil self be interested in screwing the experiment uo?
  
  Only if successfully completing experiments is itself good or evil. The question stipulates they are our moral inverse, not opposite in every way.

i ask them what happened to have things turn out like that
if they are the same person as me, and simply decided to pick the most hateful way forward each time a choice was presented to them - i'd probably turn around and try to never think of them again. Because at least for others i can understand how life could've pushed them towards the wrong path. But for myself i know that good choices were always an option, so my evil alternate self would have to consciously act out of spite and hate every time

Oh, they must be the version of me without the inhibitions, then? Cool. Let's go to town. >:-)

Kill him. There can be only one.
Oh God, I believe that makes me the evil one. What do I do?
- Killing villains doesn't make you evil. Unless you were already evil which would make your counterpart good, and killing them is just in your horrible nature.

Assuming they would have taken more or less the same life path I have, I would want to sit down and trade stories.
"What did you do back at that one time in Summer of '19? How did that go?" I guess I would hear some appalling stories, but I would be too curious.
- You considered raping or killing someone didn't you? Brave to just admit it like that.

Congratulate him on being bad and ask for advice on how not to be nervous when talking to strangers because clearly he's got to have gotten over that if he's an evil counterpart to me.

We'll probably discuss who's actually the evil one and probably have to settle the argument with our fists. Hm, maybe not a good idea, I'm kinda of a pushover, so I'd probably just accept his bullshit and see if evil me could at least have a paid spot for me in whatever shenanigans he was doing