While the truck is certainly poorly designed for pedestrian safety, the truck hit the deer at 75 mph. There is no such thing as safe for pedestrians at that speed.

It's illegal to drive a Cybertruck in the EU, because of how dangerous it is to pedestrian:

the sharp-edged, rigid stainless steel panels of the Cybertruck violate EU safety regulations, which primarily serve to protect vulnerable road users. While US safety standards focus primarily on the protection of vehicle occupants, European regulations place particular emphasis on the safety of pedestrians, cyclists, and motorcyclists.

EU regulations require passive safety features such as impact protection zones and prohibit sharp edges on the vehicle body. In addition, speed limiters are mandatory for vehicles over 3.5 tonnes – Requirements that the Cybertruck does not meet.

https://www.heise.de/en/news/Cybertruck-remains-banned-in-Germany-also-for-the-US-military-10699194.html

I was wondering how a truck could possibly slice a deer in half. The only sort of damage I could imagine from an impact at high speed was more like torn apart or mangled, unless it had like literally a blade welded at the front.

There's a video inside showing the damages on the vehicule (not the deer, it's safe to watch — there's only a few drops of blood). Turns out the impact caused a metal piece to turn and expose a sharp edge, which probably caused said slicing.

If pedestrians, like deer, are sliced in half, they’ll be safe from filing a lawsuit against Tesla for dangerous car design, design flaws in car, claimed autonomous features being lethal to surroundings, and so forth…?

In general an instant death could be said to be less cruel. The stainless steal will allow for easy cleaning and can withstand damage from foreign objects. The real victim is the car owner having to live with the consequences of killing a pedestrian but a therapy subscription to the center console will fix that.