The reason why Shiena killed the criminals if the mc escapes by his own means is a mix of many things:
- There were no soldiers or civilians from Timfalme watching, so she did not worry about the opinion of others.
- They hurted the mc and forced him to commit murder in order to survive. Sheina was already attached to him, so watching how bad the mc felt by being called a "cold-blood murdered" from the mouth of a real criminal, only made her angrier (because hipocresy).
- Thanks to the mc, she knows that there are no more kidnapped civilians in the hideout, so there was no need to investigate the place from the inside.
So basically, they hurted and made sad to her partner, and there was nobody else watching, so they had to be squished like the pests that they were to her eyes.