Show that $\ker(C)=\{0\}$ - what is wrong with this arguement? Consider the operator $C:X\to X$ where $X=C([0,1])$ and $y\in X$ where $Cy(t)=\int_0^ty(\tau)d\tau+y(t)$. I want to show that $\ker(C)=\{0\}$. Here is my arguement, but I am not convinced that it is a very good one - what is wrong with it?
Suppose that, $$Cy(t)=0$$
$$\implies\int_0^ty(\tau)d\tau+y(t)=0$$
$$\implies\int_0^ty(\tau)d\tau=-y(t)$$
$$\implies\int_0^1\int_0^ty(\tau)d\tau dt=-\int_0^1y(t)dt $$
$$\implies\int_0^ty(\tau)d\tau=-\int_0^1y(t)dt$$
Now, the upper limit $t$ in the left integral is such that it must be from the interval $(0,1]$. Without loss of generality, suppose that this upper limit is $t=1$. Then,
$$\implies\int_0^1y(\tau)d\tau=-\int_0^1y(t)dt$$
$$\implies y(t)\equiv0,\,\forall \, t\in(0,1]$$
As $y(t)\equiv0$ is the only function that could satisfy this equality. What is wrong with this arguement? Is it valid? I have doubts that it is...

There are precisely two errors in your reasoning (each of which was separately observed in the comments on your question):
The first error is that the passage from $$\int_0^1\int_0^ty(\tau)d\tau dt=-\int_0^1y(t)dt $$ to
$$\int_0^ty(\tau)d\tau=-\int_0^1y(t)dt$$ is completely unjustified. (Why did you drop the $\int_0^1 ... dt$ on the left? Are you assuming the integrand ($\int_0^ty(\tau)d\tau$) is constant with respect to the variable $t$? It is not.)
The second error is that $$\int_0^1y(\tau)d\tau=-\int_0^1y(t)dt$$ only implies that $\int_0^1y(t)dt = 0$; it does not follow that $y$ is itself constantly $0$.
All the rest of your reasoning would have been sound, but each of these steps is unsupported and in fact fallacious.
