Oh, I do like the model you used. Nice face and one of my favorite body types.

I encourage any and all improvement, so let's get to that constructive criticism bit.

First, neither character's movement suggests a murder. Slow down and consider how violently someone would struggle in Summer's position, and how difficult it would be to saw off the head of a struggling captive.

Err on the side of exaggeration.

Second, keep those characters moving. Motionless 3D characters shatter the illusion of life, especially at the end where Summer's head is hanging by a thread--yet her body stays completely still.

Similarly, the motion that is there is stiff, as if you set two keyframes for each movement and let the software auto-tween a linear path between them.

Third, do some observation and watch how all the pieces that make up a human body work together to achieve even simple motions, like typing. As I type this, my fingers are doing most of the work, but my hands, wrists, elbows, and even shoulders are moving as well depending on how far I reach for each letter and key combination.

I say this because Summer and Trey's bodies often moved one joint at a time, like PS1-era game animations. You've got some fantastic models at your disposal in SFM, not to mention a free professional-grade animation software; squeeze everything you can out of your tools. Make your viewers believe in those characters!

I won't touch on the effects much, as I know SFM doesn't have much (if anything) in the way of fluid dynamics. However, if Trey's going at Summer's neck with a knife, there will probably be some blood dripping down her chest well before her head comes off. I don't know how you can achieve this--just be aware that it's something people probably expect.

That's all I have for now. You certainly have the right idea in seeking out constructive criticism from as many people as possible--that's the fastest way to improve, in my experience. If you feel overwhelmed, break everything down into the simplest steps you can--and remember that no one starts in mastery. I mentioned that your characters' movement looked like PS1-era game animations--but at one point in time, that was professional 3D game animation quality!

For further reading, pick up The Animator's Survival Kit, by Richard Williams. It's indispensable, and every animator I've ever met has recommended it, no matter if they worked in 2D or 3D or even stop-motion clay.