Targets (for practice) and people (to kill them).
Marines must do as they are ordered by their officers; which may including killing.However they have the right to refuse certain orders that they know are criminal acts.
They don't but its their duty to kill the enemy.
marines
you shoot,grenade,or punch them
there are about 50,000 people that inlist for the marines every year
Kill the enemy, and stay alive.
No not really but if you want to you can.
The marines had to begin because they wanted to save people that hurt and graduate
Some marines in the Pacific during WW2 said it took as long as 20 seconds for it to kill Japanese.
2 is the answer
In the fluff, a 10 man squad could win a battle. Space Marines are near impossible to kill, it is acceptable to sacrifice 40 or more guardsmen to kill one marine. In the game, they are a very rounded army you can make them fit your fighting style.