I was Zalika. Let me explain my PoV last round.

At roundstart sec was barebones, there was just me and the other officer Jun. We get a report from the AI that the bartender might be a changeling. We find him in medbay and we cuff him. He transforms into a monkey but he failes to escape. We catch up to him and put an end to his life.
AI warns us chef got gibbed, I try to run a little investigation but I'm left empty-handed. In the meantime the mechanic prints out a shitton of sechuds and gives them out for free thus leading to people randomly getting set to arrest. Then the AI goes braindead, science makes xeno and not long after they "break" loose. I end up facehugged because I forgot the breath mask doesn't stop those critters from jumping on you and spend some time in medbay.
The HoS boards the station, the security staff is now 3 strong, and soon after the changeling comes to brig and transforms. I had no means (in that moment) to defeat him and I'm still learning how to robust a horrorform, so I ask the HoS what she wants to do. They say to let him roam freely but agrees to shoot him at the first occasion. I think that he might have helped fending off the xenos science so I agree.
I keep my eye on medbay while trying to catch the guy who's still setting people on arrest. Finally I found the morgue (didn't remember where it was in Meta) and only then I realize it was "filled to the brim". A few minutes after I announce it over the security channel I get attacked by the changeling and end up in medbay (I emptied a full energy gun on him and one shotgun shell and he was still kicking).
I go into surgery where the medical staff forgets they can use sleepers to wake up people and finally wake up as the shuttle was mere minutes away from departing. I got ZASed shortly after getting to the cockpit.

Not my best round to be honest, I admit I suck at investigating and ended up monitoring an innocent department. But there were only two people in sec at the start and the third joined a lot later.
You're right, it was a bad decision to let the changeling go but I also had no intention to suicide against a horror form. During the round I gradually lost focus on what was happening and had a hard time keeping track of every crime which eventually led to me making bad decisions.

I apologise for what happened but let me remind you, in the end, it's a game and people make mistakes.