Crying it out?!

My 20 month old goes through phases where he does not want to go to bed! He normally goes to sleep around 8-9 depending how late his nap was, but sometimes he just cries and cries until we get him out. Once he’s out he’s happy and playing. We want to keep bedtime routine on track so is it okay to let him cry himself to sleep? We feel terrible doing it the few times we have. He can cry for anywhere between 20-45 minutes. Usually after that we get him out. Is this cruel?!