Liquid Snake is a terrorist who looks just like Solid Snake almost.
That's because Liquid Snake is really Solid Snake's twin brother. They were both sort of "clones" of Big Boss, the ultimate soldier. They almost made Solid Snake weaker then Liquid. Basically boosted Liquid while putting Solid down. But as some sort of mental experiment they told Liquid he was the weaker one and didn't say anything to Solid. So Liquid really really hates Solid Snake.