If the largest angle is less than 90 then all the angles are less than 90, but then the triangle becomes acute angled and hence the altitude from B will lie inside the triangle and hence INSIDE the circumcircle of the triangle and as a result the altitude can't be tangent to the circumcircle. So we conclude that the largest angle is greater than 90.

now if you take the largest angle to be equal to 135 then you get a degenerate triangle(prove it).

if you take the largest angle to be greater than 135 then you don't get a triagle at all.