Help me Explain this Alien's Traits

I'm looking for biological explanations for some traits I want to give to an alien creature.

Description of the Creature

It looks like a five foot tall praying mantis, or perhaps a centaur. It has six limbs: Two big hind legs bent like a grasshopper's, along with two cat-like legs, on its lower body; and two forelimbs with huge horn-like claws coming from its upper body. It is covered in armour plates, and it runs freakishly fast (probably in the pattern of a cheetah). The creature also has three eyes on its head (two major and one minor), and two or more light-sensitive spots or demi-eyes on its torso that give it great situational and physical awareness. It is a herbivore and primarily a flight creature. It has some intelligence and tool use.

Environment: The creature lives in a temperate area of its planet, where the vast majority of creatures use toxins for defence and hunting (stingers, spit, squirters, even breathers). The armour is to prevent creatures injecting it with poison, and the speed is outrun various other predators which sometimes use enveloping tactics.

The question:

There are various questions I have for it, but the one I'll focus on for now is: I was thinking it would have a very stable upper body when it is running.

Later on, people attach some guns to its foreclaws, and find the creature is very good at running and shooting with them. But I wasn't sure why and how is the best justification for this upper body stability while running.

Possible Explanation

My first thought: It has very strong muscles up its back, and possibly air pockets particularly where the lower and upper body meet. These act as a shock absorber, and the creature keeps itself stable even while sprinting wildly.

The question being, why so much trouble to keep itself upright? An idea I considered, was if the creature was able to throw rocks with its claws. The idea is, its claw is the shape of a crescent. The creature holds the stone at the base of its claw, then as it lets go of the rock, it swings its arm up and forward. The stone is accelerated along the crescent, coming off at the end and speeding towards the target (if done while the creature is sprinting at over 40mph, that rock is going to pack a lot of punch). The claw may need a shallow indentation along the inside of it, so that the rock doesn't slip out of the claw.

That motion seems really tricky to pull off, but it's possible the creature, having claws all its life and practicing it, may eventually pull it off consistently. I thought I recalled some weapon that worked like this, but I cannot find it. The creature could use this to ward off predators and other threats/rivals from a safe distance, without risking damaging its claws (which it needs to cut trees and drink their sap).

. . .

This could explain the need for a stable firing platform, and its skill with firearms when they are introduced to its world, perhaps?

I would be interested in correcting this explanation, or hearing an alternate one.

Additional Creature Information

Here is some optional information about the creature not directly relevant for the question I asked, as it may be helpful. I'm quoting my comments form a conversation from a kind member who gave a detailed reply to my creature concept.

It seems it was necessary to clarify. The creature looks like an insect, but it isn't actually an insect. The forelimbs aren't really that heavy. The large claws are probably about a 15 inches long, so they won't be heavier than a bull's horns. The arms themselves aren't that special. The upright body gives the creature some forward weight, which might be a concern to gait. The grasshopper legs were intended for a powerful, sudden start to the creatures run, with the legs being so nimble they could get back into position for another spring in time with the front legs.

Potentially more importantly, part of the reason for the grasshopper legs was so the creature could scuttle while lying prone, exposing only its weird knees in the process. This is important for tactical reasons at one point in the story. Now, reverse grasshopper front legs may work as you suggest, and may well be a capital idea. I thought it may not be necessary, but the creature is meant to run like a devil so it may be just as well.

The tool use isn't anything to write home about. They throw rocks, and use sticks, and perform various acts similar to monkeys and crows. They can do more than would be expected with the claws, but they are still highly limited in applying their already limited intelligence. Its claws are like horns, they're only sharp near the tips. In the creature's case, they are important to its survival, it will have to rely on others of its species (who won't want it hanging around) to eat if its claws are broken. Also, they just aren't that aggressive, throwing rocks from a distance suits them better.

I can add the information. Didn't know how deep to go into the creature's physiology. The creature only cuts through a thick layer of bark to get at the sap of the trees, it doesn't cut trees down. Also, on the rocky throwing thing, it's mostly my idea for how they have these traits that become useful to the story, other alternatives are welcome possibilities.