Straight to the point - every scene in Manhunt requires fixed number of points (no less than) to achieve Max rating (5 stars). Points given for kills/executions can be expressed in simple numbers, as well. So to get the highest rating you can use different combinations of kills and red, yellow, white executions. I won't go in detail on how I figured out the exact numbers - it's based on a simple linear equation. I just present the results to you:

Kill(no matter beaten or shot) - 2 points

Hasty(white) execution - 4 points

Violent(yellow) - 5 points

Gruesome(red) - 6 points

Smashing(exploding) a head - 1 point. Can be achieved with bat, shotgun, sawn-off shotgun and sniper rifle. Doesn't matter to a dead, or a living body the head belongs to. Interesting fact1: You can smash the head of a beheaded body. Interesting fact2:

Gruesome(red) execution with a bat - 7 points

Since I've been speedrunning Manhunt's Max Rating category for a while, I can tell exactly how many points each scene requires and what combination of kills-executions to use to reach 5 stars as fast as possible. Here's the playlist showcasing it: https://www.youtube.com/playlist?list=PLDJR5SUSlZcnU0MG4jXe-JTQD4v0XPjfa It contains optimized level-runs as well as single-segment full-run.

Text version is below:

Born Again - 40 points (6 red + 1 white execution)

Doorway into Hell - 46 points (e.g. 2 red + 2 yellow + 5 white + 1 kill + 2 head smashes)

Road to Ruin - 48 points (2 yellow + 7 white + 2 head smashes)

White Trash - 75 points (e.g. 1 red + 5 yellow + 6 white + 4 kills + 12 head smashes)

Fuelled by Hate - 85 points (e.g 6 yellow + 1 white + 17 kills + 17 head smashes)

Grounds for assault - 67 points (e.g. 3 yellow + 13 white or 7 yellow + 8 white)

Strapped for Cash - 78 points (11 yellow + 5 white + 3 kills)

View of Innocence - 48 points (24 kills)

Drunk Driving - 82 points (e.g. 3 red + 8 yellow + 3 white + 4 kills + 4 head smashes)

Graveyard Shift - 65 points (28 kills + 9 head smashes)

Mouth of Madness - 82 points (6 red + 2 white + 16 kills + 6 head smashes)

Doing Time - 56 points (you always get 5 stars)

Kill the Rabbit - 97 points (38 kills + 21 head smashes)

Divided They Fall - 73 points (4 white + 19 kills + 19 head smashes)

Press Coverage - 73 points (32 kills + 9 head smashes or 2 white + 30 kills + 5 head smashes)

Wrong Side of The Tracks - 52 points (20 kills + 12 head smashes)

Trained to Kill - 71 points (28 kills + 15 head smashes)

Border Patrol - 76 points (1 red + 1 white + 29 kills + 8 head smashes)

Key Personnel - 82 points (3 red + 3 white + 26 kills or with a shotgun 32 kills + 18 head smashes)

Deliverance - doesn't matter, you always get 5 stars.

P.S. I obviously have no idea about how many points you need to achive 1-4 stars since this info is completely useless.

P.P.S. Certainly you can complete the scenes for 5 stars in a completely different fashion. For example, you can beat Road to Ruin with just 7 bat red executions and skip the other 4 hunters. But it will be slower. I tried to showcase the fastest combinations here.