Who is he
Amachi was a medical-nin and scientist that was working to Orochimaru in one of his facilities that is part of latter's Hidden Village of Sound and terrorised the people of the Land of the Sea into making them think the sacred Demon Island was cursed by a demon that spirited away innocent villagers.
What he has done
As a scientist working to Orochimaru, Amachi was a sick, twisted and ambitious scientist that used Orochimaru's resources to further his own scientific curiousity and had no interested in Orochimaru's cause besides profiting of his support but still showed respect for the latter, of course, until Orochimaru cut his resources. Settling himself in the facility of the peaceful Land of the Sea where violence and war are almost unknown, Amachi used Orochimaru's subordinates to kidnap dozens of citizens to use them as test subjects in his human experiments (some of Anko's flashbacks shows human organs inside of capsules, showing these human experiments were extremely inhuman), transforming innocent people into sea abominations in order to create an army of hybrid shinobis and sell them to all Shinobi nations so he could spread the continent's war to the oceans and evolve the art of naval warfare.
After dozens and dozens of failed experiments, he kidnapped a young girl known Sarabi from the village and transformed her into a sea beast. After the legend of a local demon kidnapping and devouring citizens had spread across the land, Amachi made sure to return Sarabi to the village, knowing she would be accused of being a monster for being the only person to have survived the demon's attack. As he expected, Sarabi was isolated from the village and was daily abused and tortured by the locals for thinking she was a monster. Under his orders, Sarabi was forced to attack ships from the region (both ships with funds for the village and basic supplies like food and medicine), kidnap more people to his human experiments after promising to return her to her normal form. In truth, he was deceiving her into thinking he was getting more results from her attacks to study the possibility of returning her to her normal human form when in fact he was merely wanting to complete his research and use it to overthrow Orochimaru as the greatest scientist in the world.
When his base was discovered by Anko's team, Amachi activated the self-destruction of his base and exploded his facility over the heads of his own subordinates inside that were fighting Naruto and his friends. When informed one stayed behind, he simply responded that they doesn't have much influence on attacking a collection of ships. After that, he attacked the fund ships of Konoha and tried to drown everyone of the crew of the ships to steal the funds and open a new facility to continue his experiments elsewhere. When Sarabi succumbed to Naruto's talk-no-jutsu, Amachi reveals his monstrous form and attempts to kill Sarabi for being useless and weak.
When defeated Amachi revealed he was intending to dissect Sarabi once she was no longer of use.
When Naruto, one of the most optimist shounen hero of all time, calls you a "rotten monster to the core", you know you are bad news. For now, easy yes.