I'm not sure it does show a failure on thorpes part? Thorpe Park's security procedure (the parts which we see anyway) is IMO probably the most stringent out of all the Merlin theme parks already and will no doubt evolve again after this incident. I agree that I haven't had to walk through a scanner at thorpe other than on some special events, but the same can be said for any entertainment venue I've visited in the UK over the past couple of years.
The bag search was bought in after a number of terrorist incidents. I agree if they want to search every guest to a higher standard it will take investment in more equipment and infrastructure, and that 2 hour queues to get in are not a long term solution, but to say the park don't have proper security measures is a ridiculous statement.
I don't agree Thorpe Park is not a safe environment, "Behaviours" you mention can and do happen in all sorts of places, when you manage thousands of people a day your bound to have trouble at some points, Fright Nights are typically busier and therefore your probably more likely to have someone out to cause trouble. And as far as I know this is the first instance of knife crime at TP - how would a "properly secure entrance" stop someone picking a fight in the park?
Holding Merlin or Thorpe accountable for the knife crime problem in the UK is abit OTT - Thorpe Park's checks were always simply Bag Checks - The park no doubt work with local police forces to plan and execute things such as security / bag searches etc.
To enter the park you need to have a ticket scanned at a turnstile or manually by staff, they of course know how many people are in the park at any given time, as far as i know there are no "legal limits" for Covid 19 - they simply have to ensure social distancing measures are in place which they are, park capacity is significantly reduced from normal.