Is a free VPN dangerous ?

I wouldn't trust that a free VPN isn't storing your data and selling it off to any third party that wants it. There's a lot of affordable options out there, with good reputations etc. But the bottom line imo is you're routing your traffic through some private company (the vpn), and they might be persuaded by some means to share your data. If you're just hiding your browsing from your isp it's probably fine. If you're trying to browse the web with more anonymity then use tor.