You haven't told me anything. You say Mirza Ghulam Ahmad is a liar and evil. Those are serious accusations. What research did you do to determine that? What evidence do you have?
As
@CG Didymus says it sounds like their aim is to produce good:
So you have the problem of two good fruits therefore two true prophets, unless you can explain exactly why Mirza Ghulam Ahmad is evil and therefore a false prophet.
The Baha'is have a similar situation with other religious groups that have a founder that claimed to be a prophet or some sort of "Promised One." The Mormons are one of those groups. Their prophet claims to have spoken with an angel from God. For Baha'is, that
should qualify him to be at least a prophet or maybe even a manifestation. But no...
Regarding your question concerning Joseph Smith and the 'Book of Mormon'; as the Bahá’í Teachings quite clearly outline the succession of Prophets from the days of Christ as being Muhammad, the Báb, and finally Bahá’u’lláh, it is obvious that Joseph Smith is not a Manifestation of God.
Shoghi Effendi, Lights of Guidance, p. 510
The Mormons are a people with high principles and ideals, and the step spiritually into the Cause is not as difficult for them as for many others not possessing their faith and devotion. However, the very zeal with which they serve their own Faith makes it difficult for them to grasp the greater vision of our Holy Cause.
Shoghi Effendi, Lights of Guidance, p. 510-511
As for the status of Joseph Smith, founder of the Mormon Faith, he is not considered by Bahá’ís to be a prophet, minor or otherwise. But of course he was a religious teacher sensitive to the spiritual currents flowing in the early 19th century directly from the appearance of the Báb and Bahá’u’lláh and the Revelation of Their Messages of hope and Divine Guidance. In this respect you might find chapter ten in the late Hand of the Cause George Townshend's book, 'Christ and Bahá’u’lláh,' interesting.
Universal House of Justice, Lights of Guidance, p. 511
So, do Baha'is really believe the claims made by Joseph Smith? Did he speak with an angel, who told him where to find the Golden Plates, that got translated into being the Book of Mormon? And do Baha'is believe what is said in the Book of Mormon?
I don't see how Baha'is could say "yes", so I'm going to assume they say, "no". And that still puts them in a difficult position... They have to accept the Mormons as people that believe in God and are doing good things, the good fruit, but reject their Scriptures and reject their founder as being a prophet.
And that's probably what they will do with the Ahmadiyya... Accept the people as being good, spiritual people... that are producing good fruit, but reject their founder's claim as being the Mahdi or Jesus or even a prophet.
So, if that's what Baha'is do, then they can reject any strange and weird person that claims to be the return of Christ or the Mahdi, but accept the people.
The trouble is... It's that person that is claiming to be Jesus or a prophet of God that is getting people to believe in him and his teachings that is getting those people to have the good fruits. Then how can we know a false prophet by their "fruits", if even they have good fruit?
And that gets us to what some Christians say about the Baha'i Faith... The people may do good things, but that doesn't make their prophet true.
It is the false beliefs and teachings of their prophet that is the "bad" fruit, because those "bad" teachings deceive people and lead them away from the true Jesus.