Hello Miki. Here are some more suggestions.
If the paragraph is describing the concerns of a couple or a family, you could continue with the use of the plural pronouns such as we or us.
To clarify the action, the cat is not GOING to be neutered of his own will. It is more accurate to say that you are choosing TO HAVE him neutered.
Often, the word LETHAL is associated with deadly things attacking from outside the body like weapons or poisons. The word FATAL is used more to describe diseases or health complications.
We have decided to have our cat neutered.
Although unsure it was the right thing to do, we know it will prevent him from spraying inside the house. Marking his territory in this way is smelly.
We hope he will forgive us for being so brutal and not suffer any fatal effects.
I hope these points are useful to you.
Regards, Naomi
P.S. - I had all three of my cats neutered without any ill effects.