Salafists, Deobandis, (some) Wahhabis, etc. don’t believe - from an Islamic perspective - in getting involved in any such (kuffar) political processes.
So it’s not a case of the following:
Islamists = those who get involved in terrorism, violent political activism, etc.
Muslims = those who don’t get involved in terrorism, violent political activism, etc.
"Islamist" is really just a specific technical term. For example, the UK’s MCB and MAB are made up of Islamists; whereas Anjem Choudary isn’t an Islamist in these senses.
In other words, an Islamist isn't just someone who wants an Islamic state and full sharia law (as Anjem Choudary does): he’s someone who may well become a councilor, MP or a member of CAIR/MCB in order to bring about sharia law (which Choudary and the Salafists would never do).
Because of all that, it's safe to say that Islamists are far more dangerous than Salafists like the street clown Anjem Choudary.