If you're too obvious about helping players, it might come off as condescending or annoying.

In New Super Mario Bros Wii, if you die on a level too many times (more than eight in a row), it makes an annoying noise and a green box appears to break. Breaking it makes Luigi appear, and he runs a ghost of one of the game's developers showing you how to get through the level.

My complaints with it were:

1) The noise was annoying, and alerted everyone else in the living room that I died eight or more times on the level.

2) Almost every time it appeared, it was because I was dying while trying to accomplish a super-hard-bonus-task, not while trying to make it through the level normally, so

It was like Clippy from Microsoft Word appearing onscreen and saying, "Hi! I see you suck at this game and are dying too much. Want me to show you how to get though the level?", when my goal wasn't trying to get through the level, but trying to get a specific hard Yoshi Coin on that level. This occurred many times, since I was 100-percenting the game.

This tutorial system is a good idea, but the game failed to tell when you were dying because the level was too difficult for you, or dying because you were getting all the bonus coins that were meant to be challenging.

I don't think rewarding a player for failing to achieve the game's primary goal (survival) makes a lot of sense. However changing the goal or the challenges is a great way to keep the player motivated and continue them on the learning curve of the game's language. This is especially useful if you can do it in fiction, like using alternate side missions when one mission is failed. As for multiplayer, I feel helping other players has to exist in the game's mechanics to be effective, frankly I find often players are too sensitive and have far too much ego on the line to simply listen to each other. The mechanics that guide a player through a single player adventure should be used in multiplayer to help players guide one another and teach one another the learning curve of the multiplayer game. RTS game ping system enables players to cooperate simply by informing and suggestion. Which is really the only way to get other players to do anything really.

I think multiplayer should have more cooperative and command elements. I've toyed with the idea of death match that evolves into team death match by having each kill add that victim to the killers "team" thus building combat groups that face off against one another until all units are united under a single player. I still haven't tested it but I like the idea that skilled players could be shadowed by newbs and perhaps even the skilled player could use tactical command controls to guide the new players into situations where they're able to achieve kills. Naturally each match would end (after unification is achieved) with a survival level of increasingly difficult AI enemies or titan enemies. Giving the group of players a chance to try and survive alongside one another.

the better player you are the more challenging the game gets (all want to kill you)

In free-for-all pvp, This has already been repeatedly exploited.

In some games a high score player ends up in one location with one or two revenge rushers who come straight towards them.

How do you mean exploited?

As a pretty good FPS player (I've been banned for "cheating" numerous times) I have a good insight on that. It's just I have no interest playing against players who are so much worse than me killing them yields me nothing. I want to succeed, I want to push my limits, I want to learn and improve like everyone else. That's why if someone manages to beat me with skill I am immediately driven to try and face him. "Finally! Let's see what this guy is made of!"

I assume the others do that on their part for the same reasons as well, at least pretty much everyone seems to draw to me. Some people prefer verbal attacks or even kickban if they have admin rights.

The phenomenon is unfortunate in this extent but in the end the behavior is all natural and thus unavoidable. If not even the driving force behind FPS gaming.

ay one player is turned into a magnet in an FPS with the intention of killing them. Normal players will see people running perpendicular, hard targets. The magnet will see players running directly at them, sitting ducks. The magnet has a better opportunity at scoring points, so the idea of magnetizing somebody to defeat them will work by making them a bigger target, but it may also end up allowing the magnet to score more points in a shorter time.

In Quakelive, there is a mode in which there is a quad-damage power-up, this power-up is dropped like an item when the player carrying it dies, and it gives it's carrier four times damage(output) and the ability to score points through killing.
All the other players are trying to kill this carrier to take the power-up for themselves and start scoring points.

Interesting detail i noticed, in the normal FFA-mode this item also exists, and only gives four times damage, still most players will go after the carrier when they can.
(in FFA it only lasts 30 seconds in total, then respawns some time later)