Generally we do not hold manufacturers liable for the malicious use of their products. If someone stabs someone else, do we hold the knife manufacturer responsible, too? If someone runs over a pedestrian, do we hold the car manufacturer responsible? If a diabetic eats pizza and dies, do we hold the pizza vendor responsible?

If weapons are marketed as being good for committing mass shootings, then they can be held accountable for that. Otherwise, no, you just don't seem to understand how product liability works.