THERE IS NO "MUST SLIDE" RULE!!!
Rule 7.08 - Any runner is out when -
(a) (3) the runner does not slide OR attempt to get around a fielder who HAS THE BALL and is waiting to make the tag.
The key phrases here are: "or attempt to get around" and "has the ball waiting to make the tag."
The runner may slide or attempt to get around the fielder. He does not have to slide. Plus, unless the fielder has the ball, the runner doesn't have to do either.
The purpose of the rule is to prohibit the runner from deliberately crashing into a defender who has the ball, for the sole purpose of knocking the ball loose, because the runner knows he is going to be out otherwise.
The fielder should not be in the base path without possession of the ball. If he is it is obstruction. Anytime a runner deliberately and maliciously crashes into a fielder he should be ejected for unsportsmanlike conduct. However, if a close play occurs and the runner does not slide and makes incidental contact with the defender before he has the ball, no call should be made. If the defender has the ball, the umpire should judge as to whether the runner made an attempt to get around the fielder. If he did, he should not be called out simply because he did not slide or made contact.
In attempting to get around a fielder who has the ball waiting to make the tag, the runner must not run more than 3 feet to either side of a line that goes between him and the base he is advancing to. If he does, he is out for violation of rule 7.08 (a) (1)
If the defender does not have possession of the ball, and a collision occurs as he steps into the path of the runner as he attempts to catch a thrown ball, there is no penalty, unless the umpire judges the collision to be deliberate and malicious.
See also; Collisions.