Is it a mortal sin to kill insects?

I just killed a spider and am wondering if it is a mortal sin.

No, it is not a mortal sin. If you had reasonable cause to kill the spider (as distinguished from killing a living creature just for the fun of it), then you did not even sin at all.

DISCLAIMER: The views and opinions expressed in these forums do not necessarily reflect those of Catholic Answers. For official apologetics resources please visit