Why does the Christian god create evil and/or catastrophes?

As for evil, Jehovah made evil as an option because love without any option is not real love. And, catastrophes are, basically, a sign that people are doing stuff they shouldn't. Such as, consider how the earth is, basically, being weakened due to all the oil being pumped out of it, which can cause earthquakes.

Since the definition of "evil" is "anything opposed to the will of God", God obviously cannot create it.

Isaiah 45:7 "I form the light, and create darkness: I make peace, and create evil: I the Lord do all these things."

God did not create evil. It is a word that was translated from the Hebrew inadequately.

Isaiah 45:7 puts forth opposites. Darkness is the opposite of light.

Evil is not the opposite of peace. The Hebrew word translated "peace" is shâlôm, Râ‛âh, the Hebrew word translated "evil" in the KJV means adversity or calamity.

Calamity is a better meaning of the word than evil.

God creates calamity to correct His children as a warning to stop doing what is wrong and start doing what is right.

it means God allows the bad times and the good times

If you removed god from the picture nothing would change, so the clergy who invented god had to claim that everything that happens in nature is evidence for god

The bible god is a mad man.

Thankfully, that lunatic doesn't exist.

Some say its because he is a vicious, vindictive, spoiled brat of a deity.

Others say because he doesn't exist and they are stories written by men.

Answered in Jeremiah 9:23-24.  AND Romans 11:33-36.  Above your paygrade.

You're right that "evil" here means inconvenience.  But it's usually not from God.  It's usually from nature, fallen existence, or from the actions of people who really knew better.

He created everything.

But as darkness is defined as the absence of light

Evil is the absence of Good

Evil is only found in the devil and his angels and the hearts of men