I made some changes, but this code is not tested but should just work the same way as before.

Question: bestill is a defined var somewhere? Better solution would be to use a flag or a skill to do this job.
Code:

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var snakemorph = 0;

Entity* snakebod;

function headhit // only do this upon impact!{my.invisible=on;}
{
	my.invisible=on;
	ent_morph (snakebod, "snakedead.mdl");
	bestill=1;
}

action snakehead
{
	// 1 is up, -1 is down
	my.health=20;
	my.gegner=on;
	var direction = 1;
	var speed = 1;
	my.enable_entity=on;
	my.enable_impact=on;
	MY.EVENT=headhit;

	while (!bestill)
	{
		walk_percent=(walk_percent+5*time)%100;
		ent_animate(me,"walk",walk_percent,ANM_CYCLE);
		
		my.x -= direction * speed;
		
		vec_set(temp,my.x);
		temp.z+=direction * speed;
		trace_mode = IGNORE_ME;
		
		if (trace(my.x, temp) != 0)
		{
			direction *= -1;
		}
		wait(1);
	}
	my.enable_entity=off;
	my.enable_impact=off;
	MY.EVENT=null;
}

action snakebody
{
	snakebod=me; // assign entity snakebod to this models action

	// 1 is up, -1 is down
	my.health=20;
	my.gegner=on;
	var direction = 1;
	var speed = 1;
	
	my.enable_impact=on;
	my.event=lev_2_event;
	
	while(!bestill)
	{
		walk_percent=(walk_percent+5*time)%100;
		ent_animate(me,"walk",walk_percent,ANM_CYCLE);
		
		// move ball in the right direction
		my.x -= direction * speed;
		
		// change direction if obstacle is in the way
		
		vec_set(temp,my.x);
		temp.z+=direction * speed;
		trace_mode = IGNORE_ME;

		if (trace(my.x, temp) != 0)
		{
			direction *= -1;
		}
		wait(1);
	}
	
	my.enable_impact = off;
	my.event = null; //MAKE THE SNAKE UNSENSIBLE FOR IMPACTS
}

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