Blood Spatter Analysis's reflexive triggered ability triggers and returns a creature card to your hand only if you sacrifice it while resolving its second ability. It won't trigger if you sacrifice it for any other reason.

Blood Spatter Analysis is sacrificed for having five or more bloodstain counters on it only while its second ability is resolving. If you put a fifth counter on it some other way, it won't be immediately sacrificed.

You don't choose a target creature card to return from your graveyard to your hand at the time Blood Spatter Analysis's second ability triggers. Rather, a second "reflexive" ability triggers when you sacrifice Blood Spatter Analysis this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.