In some online games especially MMORPG, there are unequal player killing all the time. Some player might be ambushed by other hostile players, and some who are on a low level might be slaughtered by those high level players. Some games do nothing about this and the players feel upset and leave the game. However in other deliberately designed online games, unequal player killing is a good way to evoke player emotions and competitive minds, which is of great value to keep them playing and investing. Creating different player allies to let players protect each other, keeping a "revenge list" for each player, and making those killers wanted in the virtual world... there are many good designs regarding player killing that makes the player experience actually better.
Example[]
ZhengTu is a Chinese 2D-based MMORPG. As the image shows, it has poor graphic and everything seems rough and even "ugly", however hundreds of thousands of players invest years millions in this game. In ZhengTu, unequal fight and unequal player killing is encouraged, but that does not mean that players are not protected and the game environment is unequal. Players can join a big guild to seek protection from higher level players and make them proud with more members, which satisfies both sides, or play longer, harder, and even pay real world money to buy powerful equipment to revenge because there is this "revenge list" and "wanted list" in the game to encourage killing those aggressive players.