OP, oh dear, posting this in AIBU was inevitably going to attract unconstructive responses.
I would never advocate leaving a child genuinely in distress to cry uncomforted BUT contrary to what some of the above responders say, I do believe toddlers cry for attention and your company just “because”. Not out of fear, anxiety, coldness, hunger etc (and come on, you’d know if you’d sent an 18 month old to bed hungry) etc but because they are testing boundaries, they like getting a reaction or just because they quite fancy having you there. They cry because they don't have language yet, and there are different levels of crying.
Helping them learn you won’t (can't!) be there every minute of the day or night is not being cruel as long as you give them comfort and reassurance the rest of the time. I find my toddler’s crying horrific to bear, but there came a transition where he started crying at all sorts of pointless things, like having the wrong shaped sandwich for lunch. When he’s in distress it still is unbearable but if I got upset by every crying episode I’d be a wreck.
People are projecting and being very emotive when saying the difference is "they're alone in the dark". Alone in the dark in their normal bedroom does not necessarily automatically = scared, cold, lonely!
A few questions OP. Does he like bedtime? Can he self-soothe? Do you have a routine that works? How are naps, maybe that needs reviewing?
Sorry if if this is stating the obvious, but maybe you need to "reset" bedtimes. Change existing or create a new routine that embeds a positive feel for going to bed, (along with him self-soothing). Perhaps if the CC method doesn’t work for you then a gradual retreat one might. We used CC to great effect (my child is a very, very happy sleeper, who cried less throughout sleep training than he did on an average night before we sleep trained, I strongly refute that it's cruel - he was already crying and miserable because he wanted to be asleep! ) but obviously it won’t suit every child or every stage.
I do seem to remember that 18 months was a tricky stage though for separation anxiety in general and things got loads better about 21 months. Good luck.