assalam wa alykum in islam burning any living thing is not permissible. but is it permissible to put boiling water in the corners of the house to kill insects(spiders, cockroaches etc.)? what is the ruling on the person who burns the small insects intentionally with the help of fire?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It appears –Allaah knows best –that it is not permissible to kill harmful insects by pouring boiling water over it, because it appears to us –Allaah knows best –that this is like burning with fire which is forbidden as mentioned in the Prophetic narration reported by Ibn Mas’ood . He (Ibn Mas’ood) narrated: "We were with the Prophet and we passed by an ants’ house which was burnt by fire, then the Prophet said: "It is not convenient for a human to punish with what Allaah punishes."

In addition to this, pouring boiling water is causing harm in general and does not hasten the death of the insects. Indeed, among the good traits of Islam is to kill in a good way and this necessitates choosing the means which kills the living being quickly and which does not cause it a slow lasting harm.

Allaah Knows best.