Locking my 3 y/o in his room, am I a monster?

My 3 year old son used to sleep like an angel. We transitioned him to a big boy bed about 6 months ago. He always stayed in bed and we never had an issue. Well as of last month, he has decided to stop sleeping.

He now fights bedtime and his nap. No matter how many times we put him back in bed, he comes running out.

What we’ve tried:

Baby gate : he climbs over it

2 stacked baby gates : climbed over it

New night light

New sound machine

Different sound on sound machine

New mattress

New special sheets

Laying with him (he cries the second we get up)

Reward chart

We’ve had to lock him in by switching his doorknobs so it locks from the outside.

I feel like a monster.

He screams for like 2-3 min and then lays on his bed and goes to sleep.

I always unlock the door after he falls asleep.