When you sit down for a match, either in-person or online, remember, you are playing against a real person, your opponent. That opponent's job is simply to beat you.
If you are known to be a good player, and your opponent beats you, they are probably very happy to have done so, especially if they appear to be a weaker player. It would be nothing short of mean to say anything to take that achievement away from them. Your opponent put cards in their deck to draw and play them; it would be unreasonable to expect otherwise.
Losing is an important part of the game. Speaking for myself, if I were to never lose, putting aside financial incentives, my interest in Magic. would quickly evaporate. A large part of the fun of Magic, but games in general, is the challenge. The same way that without sadness, the comparative emotion of happiness would cease to exist, without losing, the comparative state of winning would cease to exist.
One of the key originations of “salt” in Magic stems from someone believing there was an unequal distribution of “luck” in the match. In Magic, the amount of "luck" from any given draw step is often inversely proportional to that player's "luck" for the rest of the game. For example, if your opponent top-decked their answer at the last possible moment, over how many looks did they miss? Did they engineer the game to allow themselves this out? How many things had to go somewhat "unluckily" for them to put them in that situation to begin with? You shouldn't zoom in to one instance in a game and look at "luck." It doesn't mean anything.
Likewise, looking at luck at all is not healthy, for the same reasons. Simply put, it doesn't matter. We are not entitled to win this game, or any game. Our opponents are not some faceless obstacles in the way of our own victories. Each opponent is as much of a person as you or me, and no one a “deserves” to win.
What we do all deserve is a respectful and fair opponent. This is what we owe to each other. We all deserve an opponent who respects us and respects the game, and we must offer our opponent the same courtesy.
If you get frustrated sometimes, that is okay. If you believe you got unlucky at times, that is also okay. What isn't okay is using your opponent as the target for these frustrations. Again, their objective was to beat you. They succeeded. This isn't some random NPC standing in the way of your tournament victory. They are just as real as you or me. They are happy to have won, and the better of a player you are, the more this is true! Don't take this away. Be gracious, be a good sport. Say “good games,” wish them the best, and move on.
An aside on “good game”: The phrase has an incredible amount of baggage in the gaming community, because a lot of games are won by concession instead of a literal by-the-book victory. In another game I used to play competitively, Starcraft II, practically every game was won via concession. The actual game only truly ended when every last building a player owned was destroyed, but typically, games ended much sooner than that, since a player would concede when they believed they no longer had a chance to win. This naturally led to the move known as the “aggressive GG,” which is when a player who believes they are imminently going to win uses “Good Game (or GG)” to say, “This game is over, and you have no chance of winning. You should save us both time and concede.” Obviously, this is very rude, but some people extrapolate that to think that any time a winning player says, “Good game,” it is therefore rude. This is bad reasoning. The reason that the “aggressive GG” is rude is that it's telling the opponent that they should concede. Either player saying “good games” after the match's conclusion is an acknowledgement of their opponent and that a fair match of was played. Also, keep in mind, “good games” does not necessarily mean “close games.” Good games can be close, but they can also be blowouts. Good games are fair games, and if neither player cheated, and all players were courteous to each other, then the games were good.
After the match, if you want to talk about specific lines of play, feel free to do so, but don't be accusatory. If you think your opponent misplayed, don't say it like that. Ask an open-ended question: “Hey, so at the time when you cast , but you knew I had , I was wondering what your reasoning was behind doing that?” Or “I saw that you had in after side-boarding. It definitely blew me out in that game just now, but I've noticed it hasn't been super strong against me, in general. What have your experiences been with it?”
Remember, your opponent, in paper Magic or digital, is as much of a person as you or me. Treat them, and the game, with respect. This is what we are all entitled to. This is what we all deserve.