Yup, xeno ethics stops the choices. If you choose evil, allows you to extort money from trade, free evil tourism, free planetary invasions, secret police projects, and VERY powerful weapons only evil may get (but not as good as doom ray, which rocks). Neutral gives a +1 advantage in invasions, neutrality learning centers (the most powerful research center), more trade routes (money) more tourism from neutrals, free terraforming, happier people. While you are good, people like you more, VERY powerful defences, good tourism, more likely to surrender to you, loyal people.
As for weapons techs, just go up laser. You've seen those nice purple beams the DL shoot at you? Doom ray. Great fun.
Oh, and if you give away all planets but one, the planet will be lost in a culture takeover. Game over.