Blackbeard didn’t only kill Thatch after the latter ate the Yami Yami no Mi devil fruit, Blackbeard NEEDED Thatch to eat the fruit.
1-Marshall D. Teach “Blackbeard” shares part of its name, its moniker, its appearance and many behavioral / personality traits with the real life pirate Edward Teach/Thatch known as Blackbeard.
2-Edward Newgate “Whitebeard” shares part of its name with the real life Blackbeard too. Former 4th commander of the Whitebeard Pirates Thatch also shares part of real-Blackbeard’s name.
3-Teach possesses the powers of the Yami Yami no Mi (Darkness Special Logia). The Yami Yami no Mi belonged at some point to Thatch.
4-Teach possesses the powers of the Gura Gura no Mi (Quake Paramecia). The Gura Gura no Mi powers were in possession of Whitebeard immediately before Teach acquired them.
5-Teach needed Whitebeard’s body in order to get the Gura Gura no Mi powers.
6-When the user of some devil fruit’s power dies, those powers return to the closest compatible fruit, that becomes a devil fruit.
7-Teach, a long with his crew, went from Impel Down to Marineford without stopping by at any Greengrocers nor farms.
8-Teach is able to develop wolverine-like claws, seen here (bottom right panel).
9-Teach was able to scar Shanks’ eye in a claw like fashion.
10-Teach’s jolly roger has 3 skulls and 3 crossed bones.
11-Teach carries 3 guns.
12-Teach lacks 3 teeths. And he has 3 beard braids.
13-A very strange insight from Luffy and Zoro at the same time states that Teach are multiple people.
14-Charlotte Linlin acquired the powers of the Soru Soru no Mi immediately after eating a Croquembouche, the rest of the banquet, all the other kids under mother Carmel’s care and mother Carmel herself who was until then the possessor of those powers. The order of eating is unknown as well as the composition of the rest of the banquet.
1-The claws Teach can develop are a feature of him possession the powers of a zoan devil fruit.
2-That devil fruit is a Cerberus Mythical Zoan.
3-That devil fruit allows the user to “fuse” in some way or “add” up to two other people to oneself (say like Cell did with A-17 and A-18 in Dragon Ball Z).
4-Thatch ate the Yami Yami no Mi.
5-Teach incorporated Thatch into himself after the latter ate the Yami Yami no Mi.
6-Teach incorporated Whitebeard into himself after killing him. (Or while he was still agonizing in his last moments almost dead to be more precise.)
7-Incorporating the “essence” (soul, spirit, life force, X, whatever) of an ability user that by this process ceases to exist as an alive individual “captures” the powers of the devil the same way eating a devil fruit does.
8-As a consequence, Teach acquired the powers of the Yami Yami no Mi through incorporating Thatch and the powers of the Gura Gura no Mi incorporating Whitebeard.
9-Teach could possess additional devil powers BECAUSE the holder was still another person and not just one person possessing multiple powers.
1-Teach would possess the powers of 3 devil fruits. One of each main class.
2-Teach would be multiple people. Specifically, 3 people currently, 2 people at the time Luffy and Zoro noticed.
3-Teach is one of the characters that has changed physically the most over time. And every time he’s prooved to have a new devil power his appearance has become more similar to that of real life Blackbeard.
4-The 3 characters that are named after real life Blackbeard would be one and the same, as it makes no sense for 3 different Blackbeards to exist.
*Theory by The_Real_Katakuri