Killing yourself in Counter Strike: Global Offensive has been an illegal strategy in competitive play for just a few days, but it's already set to be changed back to the way it was before. Where an update to the Intel Extreme Masters and ESL rulebook initially saw it illegal to deny kills to the other team by using the Kill command, or intentionally killing yourself in some manner, it proved so unpopular that it's now been reverted.
The reason it was a viable strategy in the first place, is because suicides lead to less money for the other team, giving them less reward for a victory than they might otherwise receive and therefore giving the suiciding player(s) a greater chance to get back into the game.
The reason the new rule was initially implemented was because tournament organizers felt that it made the game less fun to play and to watch. They apparently wanted to “discourage actions that negatively affected the competition based on recent experiences and community feedback in the German ESL National Championship," as per PCGamesn.
They now plan to put further consideration in to any further changes and will take greater care in discussing such moves with the community first.
“After a careful internal review, we are reverting the decision,” IEM states on its website. “We will engage in a deeper discussion about the rule in question with the Counter-Strike Professional Players’ Association to ensure that all efforts to improve the viewing experience for fans at home are fully aligned with all relevant stakeholders.”