"Staying together is the best way to prevent an assasine to kill someone"

Typically, standing next to someone with CC is a very good way to stop Katarina, yes. Carries aren't well known for their overabundance of interrupts.

Granted he has lower burst than Kat , but he can get going with his resets just as nicely as kat does given the right circumstances.

save me pliss...

PENTAKILL

Grouping against a Katarina isn't a bad idea as long as you have someone you can rely on interrupting her. Grouping as 5 squishies with no/bad CC is a terrible idea because Kat is THE team fight assassin but I'm pretty happy to roam around with someone I can expect to stop her in her tracks, or follow people around if I'm the person who can do that.

That said Kat is unique anyway because her power budget is based on her melting up to three people with Death Lotus at a time, which DOES make her a bigger grouping threat, but makes her a lot less scary 1v1 (an ADC will die to her but as a mage a Katarina running up to you is infinitely less scary than almost any other assassin).

Grouping is good against assassins in general, but that doesn't mean an assassin who breaks the mold is unhealthy as long as they gain a weakness in another place and Kat does - she's worse 1v1 than almost every other assassin. I think you'll see a similar thing with Ekko - he has a variety of tools that make grouping against him seem like a terrible idea.