The alphabetical sorting doesn't work if I have entries with only an organization. The bibliography should be sorted alphabetically in a non-hierachical way between the organization and the author.
I don't think that the solution with corporate authors in double brackets works, because I need the organizations in italics.
\begin{filecontents}{\jobname.bib}
@online{musk,
author = {Elon Musk},
title = {Tesla Model X},
url = {https://www.tesla.com/modelx?redirect=no},
date = {2017-07-04},
organization = {Tesla},
urldate = {2018-10-16},
},
@online{b,
title = {Tesla Model X},
url = {https://www.tesla.com/modelx?redirect=no},
date = {2017-07-04},
organization = {ABZ},
urldate = {2018-10-16},
},
@online{a,
title = {Tesla Model X},
url = {https://www.tesla.com/modelx?redirect=no},
date = {2017-07-04},
organization = {ZBA},
urldate = {2018-10-16},
},
\end{filecontents}
\documentclass{article}
\usepackage[style=authortitle]{biblatex}
\addbibresource{\jobname.bib}
\DeclareListFormat{organization}{%
\usebibmacro{list:delim}{#1}%
\mkbibemph{#1}\isdot
\usebibmacro{list:andothers}}
\DeclareBibliographyDriver{online}{%
\usebibmacro{bibindex}%
\usebibmacro{begentry}%
\usebibmacro{author/editor+others/translator+others}%
\setunit{\printdelim{nametitledelim}}\newblock
\printlist{organization}%
\newunit\newblock
\usebibmacro{title}%
\newunit
\printlist{language}%
\newunit\newblock
\usebibmacro{byauthor}%
\newunit\newblock
\usebibmacro{byeditor+others}%
\newunit\newblock
\printfield{version}%
\newunit
\printfield{note}%
\newunit\newblock
\usebibmacro{date}%
\newunit\newblock
\iftoggle{bbx:eprint}
{\usebibmacro{eprint}}
{}%
\newunit\newblock
\usebibmacro{url+urldate}%
\newunit\newblock
\usebibmacro{addendum+pubstate}%
\setunit{\bibpagerefpunct}\newblock
\usebibmacro{pageref}%
\newunit\newblock
\iftoggle{bbx:related}
{\usebibmacro{related:init}%
\usebibmacro{related}}
{}%
\usebibmacro{finentry}}
\begin{document}
Let's cite! \footcite{musk} \footcite{a} \footcite{b}
\printbibliography
\end{document}
With this MWE I get the following output: 
However, it should be sorted as follows:
ABZ [...]
Musk [...]
ZBA [...]
How can I achieve that?

authorfield but with anorganizationfield, you may want to rethink the organization (pun intended) of these entries: change the name of theorganizationfield toauthorand encase theauthorfield in double curly braces. That way, you inform biblatex (or BibTeX) that it's dealing with a "corporate" author, not the name of a "normal" person. – Mico Dec 15 '18 at 08:53