Because there was no reason to let them live. The plan was suggested by demiurge to set things up but there was leeway for these group.
It was optional for them, ainz warned them and asked their intentions.
They already found gold and treasures at the entrance so they could have walked away at that point. Unlike adventurers, they are more of the under the table group. Their task was mainly investigating the ruins, no one expects a fully detailed report. They could have said that it was old, full of undead and it would still pass for their level of expertise.
The only reason you feel bad is because you know their reasons for doing it and the people they left behind. Ainz doesn't know any of this.
The most notable thing he knows about them is they are doing this for money.
So... Anime watchers and LN readers would have know the outcome already. I did both so I guess the manga will be the same. For anybody who wants to know.
Every party that went into the ruin out of their own volition did not return. The Foresight party members all met terrible fates. Which lead to terrible fate for the twins as well.