"H" is for Heat

Here's what I think, humbly.  Disclaimer: I am not a physicist nor a theologian.

Hot (adjective) and cold (adjective) are opposites. They describe relative amounts of heat energy.

Heat (noun) and cold (noun) aren't opposites.  Heat is a form of energy. Although we use it as a noun ("Come in from the cold"), cold isn't really a thing; it doesn't exist except as the absence of heat. You can generate heat, but you can't generate cold. You can build a heat ray, but you can't build a cold ray. Your freezer doesn't generate cold; its mechanism continually removes heat from its interior.

Light is electromagnetic radiation. Darkness isn't the opposite of light, it's the absence of it.  There are light rays.  There are not dark rays.  You can't turn on the dark with a switch.  Light dispels darkness; darkness cannot dispel light.

There is good and there is evil. God is good, therefore, whatever is good has its source in Him and is an imperfect reflection of His person and character.

Evil is the opposite of good, and evil is a thing, it has "substance," it can be palpable.  But God did not create evil. God created man in His image; that is, in a finite way, he imparted his attibutes to man. One of these attributes is moral agency. God made man a free moral agent. Man, in exercising his free moral agency, rebelled against God. Evil comes not from God but from man. All evil has its source in the corruption caused by man's rebellion. Evil is man falling short, sometimes tragically so, of the good created by God.

By my reckoning, if you are asking how a good God could allow evil, you're asking the wrong question. What you should be asking is why God created man with free moral agency.