A sympathiser is someone who understands or shares the feelings, opinions, or beliefs of another person, especially someone who is experiencing difficulties or challenges. Full definition
A «eulogising profile» of Jihadi John appeared in the magazine which was shared online late Tuesday by sympathisers of the Islamic State group. (businessinsider.com)
«You should not be walking through the lobbies with Jeremy Corbyn and a bunch of terrorist sympathisers,» the prime minister told the gathering, according to The Guardian. (buzzfeed.com)
A terrorist sympathiser in other words... (politics.co.uk)