Assuming that Putin's statements are representative of the Russian people is clearly incorrect.
Invading another nation isn't "conducting security".
You can use your line of reasoning to justify any action however evil or wrong as long as the person taking it is giving a reason in the meantime.
The beliefs are due to propaganda. Most Russians have close ties to Ukraine and would never think of waging war for no reason.
OP: How to argue that invading Ukraine is wrong?
You: Here's what Putin and his supporters think.
Sure, he's made public statements about this, I don't get how they're relevant here given his position. Every tyrant and aggressor in history has had a bullshit excuse and a group of supporters. That doesn't make their position equally good to the people they're opposing - and that's what I thought you were saying with your previous comment.