I am a Muslim who came to US in 1984. During my 18 years of living here, I found it easier for me to use my middle name and shortened my last name to ease transactions. My full name was Zouhir Omar Abdul Baki, and after becoming a US citizen, I changed it to Zouhir Omar Baki. My children's last name is Baki as well. Is this sinful and against God's will?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no harm in abbreviating the name Abdul Baki to Baki since there is no evidence in the Qur'an and sound Sunnah proving that Baki is among the names of Allah, as confirmed by a group of Muslim scholars.
Using any name that could be considered one of the attributes of Allah is forbidden; the name al-Baki is not among the Names of Allah.
Allah knows best.