So you don't assign a "good" or "bad" value to a certain act and/or don't think morality exists at all?

"good" and "bad" are meaningless to me. When I hear them or read them in something, I usually just associate them with an average of what people believe the words to mean. But to me they are meaningless. As for "morality", yes, there is the word. But again, I think that it's all subjective. There's no "hard" morals.

but your actions seem to hold no emotional weight, which feels really foreign to me.

Exactly. If Mr. X happened to be someone I closely related with, I'd wager that I'd feel the loss of a friend. Losing someone to talk to or bounce ideas off. I suppose I'd have "emotional weight" then. But only because I had lost an asset.

How would you say you would feel after killing a man?

Had I thoroughly and satisfy-ably weighed the pros and cons, how I feel after would be dependent on how "good of a deal" I got. If things just barely scraped by, I'd be happy, but maybe the pros didn't outweigh the cons. However, if the pros outweighed the cons by a lot, I'd be pretty thrilled with my decision. In day to day life, it's hard to get an exact idea with how everything will turn out, so there's varying responses. Flawed data leads to unexpected results. Granted everything was in my favor, I'd have no problems. Well, aside from the cons that I decided were worth it. That is, losing a friend might be worth it if I figured the pros were definitely good enough. Different pros/cons will have different weights. But that's just all preferences and opinions. Nothing to do with the emotional attachment or "morals" behind it.

A less extreme example would be chocolate vs vanilla ice cream. I like ice-cream, and both flavors are equally beneficial. There's no real reason to go with one over the other (pros/cons wise). So it's a matter of taste. I'd weigh my taste preferences (what "tastes" better to me) and then decide. unfortunately chocolate vs vanilla is pretty difficult and I tend to switch every time. I don't feel any less morally wrong/right for my choice.

For me, ending a life would instinctively feel bad, which makes sense because I think a lot of my morals come directly from instinct.

Ending a life has an inherent con attached to it. That there is one less mind to solve problems, reason, and function in the same space. This may be a pro or con depending on the result. If the person is important, like a scientist or a doctor or something, it'd be a much bigger hit to my wellbeing than say a serial killer (who I'd be glad is gone). If it's just me vs the serial killer in an empty space. I have 0 problems with it. I'd rather not die.

In my mind, I would be able to rationalize my actions because Mr. X was an evil man, I just earned 10 million dollars and possibly saved lives. However I would probably still feel bad for ending a man's life.

So you did something that (in your mind) is morally wrong, just to benefit yourself. I fail to see what makes it morally wrong. Besides your thoughts, there is literally nothing saying this was wrong or right. You did it to benefit yourself. Just as other animals kill to benefit themselves. Is a wild animal being morally wrong, just because it wants to eat and survive?

So what would you call that instinctively bad feeling I experience?

Regret. Regret that you've made a permanent decision that, although benefits you, has gone against everything you've known about the pros/cons of killing people. It's an outlier in your data, so you naturally think about it and wonder if you made a mistake. At least, that's the feeling I get when I "feel bad" about something.

There's no undo button for killing. So whatever pros/cons there are, you have to accept them. Misjudging them, or perhaps not considering them to their fullest, can cause regret, disappointment, and a whole bunch of other negative feelings and emotions.

I get the feeling a lot, because I'm bad at judging what my body needs for nutrients and taste. I decide to get something to eat and naturally feel bad, because I hadn't weighed my options carefully enough. And sometimes there isn't an optimal choice (they all are around the same pro/con ratio).

If there is a different feeling you are talking about, it's foreign to me.