How to draw a proper decision tree

Here's a solution with forest.

Output

Code

\documentclass[margin=10pt]{standalone}
\usepackage{tikz,forest}
\usetikzlibrary{arrows.meta}

\forestset{
    .style={
        for tree={
            base=bottom,
            child anchor=north,
            align=center,
            s sep+=1cm,
    straight edge/.style={
        edge path={\noexpand\path[\forestoption{edge},thick,-{Latex}] 
        (!u.parent anchor) -- (.child anchor);}
    },
    if n children={0}
        {tier=word, draw, thick, rectangle}
        {draw, diamond, thick, aspect=2},
    if n=1{%
        edge path={\noexpand\path[\forestoption{edge},thick,-{Latex}] 
        (!u.parent anchor) -| (.child anchor) node[pos=.2, above] {Y};}
        }{
        edge path={\noexpand\path[\forestoption{edge},thick,-{Latex}] 
        (!u.parent anchor) -| (.child anchor) node[pos=.2, above] {N};}
        }
        }
    }
}

\begin{document}
\begin{forest} 
[$x_2$, tikz={\draw[{Latex}-, thick] (.north) --++ (0,1);}
    [$x_1$
        [1] 
        [0] 
    ]   
    [$x_3$
        [$x_1$
            [1] 
            [0] 
        ]   
        [0] 
    ]   
] 
\end{forest}
\end{document}

\documentclass{standalone}
\usepackage[T1]{fontenc}                
\usepackage[utf8]{inputenc}             
\usepackage[english,italian]{babel}
\usepackage{tikz}
  \usetikzlibrary{shapes,arrows,fit,calc,positioning}
  \tikzset{box/.style={draw, diamond, thick, text centered, minimum height=0.5cm, minimum width=1cm}}
  \tikzset{line/.style={draw, thick, -latex'}}
\begin{document}
    \begin{tikzpicture}[auto]
        \node [box]                                    (x3)      {x\ped{3}};
        \node [box, below=0.5cm of x3, xshift=-3cm]    (x1sx)    {x\ped{1}};
        \node [box, below=0.5cm of x3, xshift=3cm]     (x2dx)    {x\ped{2}};
        \node [box, below=0.5cm of x1sx, xshift=-1cm]  (x2sx)    {x\ped{2}};
        \node [box, below=0.5cm of x2sx, xshift=1cm]   (A2sx)    {A\ped{2}};
        \node [box, below=0.5cm of x2sx, xshift=-1cm]  (A1sx)    {A\ped{1}};
        \node [box, right=1cm of A2sx]                 (A3sx)    {A\ped{3}};
        %
        \node [box, below=0.5cm of x2dx, xshift=1cm]   (x1dx)    {x\ped{1}};
        \node [box, below=0.5cm of x1dx, xshift=-1cm]  (A2dx)    {A\ped{2}};
        \node [box, below=0.5cm of x1dx, xshift=1cm]   (A3dx)    {A\ped{3}};
        \node [box, left=0.5cm of A2dx]                (A1dx)    {A\ped{1}};
        %
        \path [line] (x3) -|         (x2dx);
        \path [line] (x3) -|         (x1sx);
        \path [line] (x2dx) -|       (x1dx);
        \path [line] (x2dx) -|       (A1dx);
        \path [line] (x1dx) -|       (A2dx);
        \path [line] (x1dx) -|       (A3dx);
        \path [line] (x1sx) -|       (x2sx);
        \path [line] (x1sx) -|       (A3sx);
        \path [line] (x2sx) -|       (A1sx);
        \path [line] (x2sx) -|       (A2sx);
    \end{tikzpicture}
\end{document}