Entire function with uncountably many zeros

Lets consider open ball $B_n$ at center zero and radius $n$, a natural number, then $\mathbb{C}=\cup_{n\in N} B_n$. Now if none of $B_n$ contains uncountably many complex zeros of $f$, zeros of $f$ will become countable, a contradiction. So suppose $B_k$ for some natural number $k$, contains uncountably many complex zeros of $f$. Hence zeros of $f$ has a bounded uncountable subset in $B_k$. By Bolzano-Weirstrass theorem it has limit point. Now by Identity theorem in complex analysis $f$ is identically zero.

As John already pointed out, an uncountable subset $A\subseteq\Bbb C$ has necessarely an accumulation point. Then the identity principle for holomorphic functions, says that a function which is $0$ on $A$, is necessarely $0$ on its whole domain (which is $\Bbb C$ if your function is entire).