TX real estate law, property code 92.0561

To fully and accurately answer your question I would need more facts and would need to see a copy of your lease, but potentially you could terminate your lease so long as the dead animal in the wall is a "condition that materially affects the physical health ... of an ordinary tenant" and that this condition was not caused by you or your guests. A call to the local health inspector may get you a report in your favor, so would an opinion from a doctor stating that your ailments are caused by decaying animal parts inside of your wall. Has the landlord contacted you at all regarding this incident? Have they responded in any form to your two written notices? If they have not, you may be entitled to terminate the lease under 92.056(e)(1), but if I were you I would not do so until contacting an attorney and having everything thoroughly reviewed for your individual case.