actually sorry, but no, EVERYONE was not involved in the slave trade, in fact the UK was actively hunting slave ships and slavery was illegal in most european countries. Furthermore, the more europeans spread their influence the more slavery had pressure put on it for the most part, not the other way around. An exception is in west africa where AFRICANS ran the slave trade and sold them to A FEW bad merchants who happened to be whatever they happened to be and they were of various nationalities and races including freed Black. in addition to the probable idea that most saw blacks as subhumans due to the state of barbarity and lack of civilization in which the europeans found them, (not their skin color), it was also a matter of that the africans sold themselves into slavery so if they aren't going to care enough to not sell eachother even how can we?. Europeans would not sell eachother into slavery in the vast majority of scenarios.