Davis had a long criminal record, even before his radicalization led him to travel to Syria and establish one of the cruelest IS cells. His arrest is an important step in holding him accountable for his crimes and demonstrates the government's commitment to prioritizing the safety and security of the UK.
Davis should've had his British citizenship revoked and been banned from entering the UK, as others including Elsheikh and Kotey have been. He's a dangerous terrorist and his stay in the UK means unnecessary costs and threats to national security.
The allegations against Davis are just that - allegations, which he has long denied. The entire situation involving the accusations against Davis and his initial arrest in Istanbul were murky, and his UK trial could go in any number of unexpected directions. Judgement should be reserved until after he has exercised his right to a fair trial.