First of all, bcd has nothing to do with A's statement.
Let's not think about a.
Only one person told the truth.
Assuming that B is telling the truth, then D is the murderer, C is lying, and D is lying. They have no enmity, that is, they have not wronged him, that is, he is the murderer, that is, A is lying, that is, A may be the murderer at the scene, and that D is the murderer, so what B said must be a lie.
Assuming that C is telling the truth, then B is the murderer, B is lying, D is not, and D is lying, then B has not framed him, so bd is the murderer, so what C said is definitely a lie.
Suppose what D said is true, what B said is false, and what C said is also false, excluding the suspicion of bd, and what A said is not true, then A is the murderer, then what D said may be true.
Suppose A is telling the truth and bcd is lying, then B is lying, D is the murderer, and D is saying that B framed him. If it is false, it is not framed, so it is not established.
To sum up, only what D said is true, then the murderer is A.