You wrote that there were a risk to loose btc using this option because of:
1. Because of malware/spyware. OK, I understand this.
2. Because of myself. If I mess up something.
Could you please tell,
How can btc be lost if I mess up something when:
- To import private key one must do a basic 10 seconds operation: Copy private key in old wallet and paste it to new wallet.
- As long as old wallet is not deleted, and old private key is not deleted, nothing can be lost even if I do importing private key completely wrong.
There are endless possibilities that can happen: you could paste your private key into an email, post it online, someone could see it on your screen, enter it into a compromised wallet (again: many fake Electrum wallets out there), etc.
Just
assume someone's out to get your money, and
assume your computer isn't safe. Some paranoia helps

Here's another real-life scenario that happened to a clueless and innocent Newbie:
ScenarioSomeone has a problem with a wallet, comes to Bitcointalk, creates an account, asks a question, and gets a solution. Great! A happy new user for the forum, and another happy Bitcoin user, which adds to Bitcoin's popularity.
Reality (2 days ago)
Someone has a problem with a wallet, comes to Bitcointalk, creates an account, asks a question, gets "help" by PM from someone who asks him to enter some code into Electrum, enter his password (and I
quote: "(NEVER share this password with anybody)"), and gets scammed out of $30,000.
Another clueless naive new user bites the dust, the forum loses a new user, and Bitcoin loses a potential user forever.