Yes, as described in the previous posts, your bonus is definitely income associated with your job. Thus, when you report this as self-employment income, you will pay income tax and self-employment tax (SE tax is your Social Security and Medicare taxes). Yes, all those who report self-employment income pay income tax and SE taxes.
The better option for you would be for your future employer to report this signing bonus on a W-2; however, if they were to do so, the employer would pay their respective share of the SE tax and you would pay your respective share of the SE tax. Filing as self-employed, you pay both parts of the SE tax--15.3%--in addition to the income tax. While it is true that you do get to deduct one-half of the SE tax from your income, that deduction only reduces the amount of income subject to tax, it does not reduce the tax per se as a credit would.
If you were to characterize this income as "other income," (an option we do not recommend) you would only pay income tax at your ordinary rate. You would not pay the SE tax. Thus, the tax difference between reporting this income as self-employment income versus other income is significant.
@Shen924
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