This is directly from the 2011 NHFS Rulebook; I see this question repeatedly asked on the site and the reality is while there is not a "must slide" rule sliding takes out most of the question/judgement regarding "malicious." Sure there can be malicious sliding tactics as well but to be safe on collision plays at home it is best for the runner to slide or give themselves up. After that is is absolutely the umpire's judgement call.
MALICIOUS CONTACT
• Half of baseball ejections deal with malicious contact by the offense or defense (ball becomes dead).
• Over half of our baseball ejections every year pertain to malicious contact. Coaches need to visit with their players and inform them
what malicious contact is and regardless what they see on TV it is not permitted at the high school level. The majority of the time, malicious contact occurs between third base and home plate, but could occur and be enforced at other bases. The runner who creates malicious contact is ruled out and ejected.
• Keys to malicious contact & player ejection
• Player leads with forearm or lowered shoulder in attempt to take out the catcher
• Runners choices (when catcher has the ball)
o Legal Slide
o Stop and return to previous base (get in rundown)
o Let the fielder tag him
• What your runner should do if a fielder is in the baseline without the ball
o Go around the fielder - umpire should call out obstruction and throw out left arm, just like any other obstruction situation
o Coaches, teach your players not to block any baseline without the ball or they are subject to being called for obstruction
o If the ball arrives just prior to the contact and runner is tagged and there is a collision you have an out, but the player is not ejected unless he leads with the forearm or shoulder
Rule 3-3-1n: A coach, player, substitute, attendant or other bench personnel shall not: initiate malicious contact.
Rule 8-3-2: Malicious contact supersedes obstruction.
INCIDENTAL CONTACT vs MALICIOUS CONTACT
A) Not all collisions are malicious
B) A violent collision is not automatically considered malicious
C) Umpires shall rule on player’s intent:
1. Contact is result of intentional excessive force, or,
2. Contact is a result of intent to injure
D) Runners should be instructed to help eliminate the question of malicious or incidental contact by:
1. Sliding legally, and
2. Attempting to avoid making contact with defensive player