Don't use it, but keep it for now just in case. Different states have different laws and procedures. Your insurance company may or may not reimburse you for a new seat, and some companies collect car seats after a crash.
When all that has been worked out and you still have the seat, yes, destroy it and throw it away. Cut the straps, remove the cover and write in permanent marker in bold letters on the seat shell "DO NOT USE: in a crash" or something to that effect. The object is to destroy it so that no one can pick it out of the trash later to use it.
How are you all doing? Is your DD freaked out? {{{HUGS}}}